Rashi interprets the word לקדשו (כח:ג) as to initiate Aharon into the priesthood by means of the clothing. We can infer from this that according to Rashi, the wearing of the garments are not a Mitzvah in themselves but a way of performing a Mitzvah.
Rambam, in his Sefer HaMitzvot (מצוה לג), counts the Kohen's wearing his garments as a distinct Mitzvah and not as a preparation for a Mitzvah. Ramban, however, in his commentary to the Sefer HaMitzvot, disagrees with Rambam, saying that even Rambam would admit that except during the Service in the Beit HaMikdash there is no separate Mitzvah to wear the garments, hence they are only a preparation or an integral part of a greater Mitzvah. It seems that in Rambam's count of the Mitzvot in the Torah, he does not count components of Mitzvot among the 316. If he did, he would have to count the fashioning of the Shulchan as a completely separate Mitzvah which he does not.
The Megillat Esther demonstrates that Rambam does sometimes count components of Mitzvot as Mitzvot themselves. Slaughtering the Korban Pesach, for example, is not a Mitzvah in itself, rather, a preparation for offering the Korban. The same applies to the Parah Aduma. The slaughtering of the cow is not in itself a Mitzvah, but a preparation for the sprinkling of the ashes. However, Rambam counts both of these in his list of Mitzvot. It is therefore likely that the Kohen's clothing are included as distinct Mitzvot even though they are only a preparation.
Lev Sameach, by contrast, holds that according to the Rambam, the Kohen's wearing the garments is a totally separate Mitzvah unconnected to what follows. He brings proof by saying that the Torah describes the purpose of the clothing as being (כח:ב) "for glory and majesty" and not to serve. Lev Sameach brings further support from Hilchot Me'ilah. There, Rambam rules that a Kohen who dresses in priestly garments for secular purposes is guilty of Me'ilah -- misuse, since the garments are meant for the benefit of the Kohanim and not for the service of Hashem in the Mishkan. If the garments were part of the Mishkan service, then the Kohen would be guilty of Me'ilah. We sse that for the Rambam, unlike Rashi, the Kohanim wearing the בגדי כהונה is an end by itself, and not merely a means to an end.