p54: rename prism54xyz -> p54xyz

It's been a long time, but fullmac prism54 driver is still around...
I think we should rename every prism54* in order to avoid some
confusion about "what is actually what" in the future ;-).

Thanks-to: Maxi <maxi@daemonizer.de>

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>