cm4000_cs: use bitrev

Cleanup using bitrev8 in cm4000_cs driver.

Cc: Harald Welte <laforge@gnumonks.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>