bus_type has been added to MediaRoster.h and the serial driver.
It is not used enough to be in any shared header.
media_roster only uses it for buffer size estimation.
Change-Id: If4f372d44e871230da4744d99ec7cde0c79c8344
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5209
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>