b43: use the bitrev helpers rather than rolling a private one

The 4-bit reversal flip_4bit is replaced with the bitrev helper
bitrev8 and a 4-bit shift.  The B43_WARN is moved to the location
where a register is read from for checking there.  The other caller
explicitly passes an array index which is guaranteed to be within range
and so a B43_WARN is not added there.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Reviewed-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>