staging: rtl8192u: rename *BitMask variables in r819xU_phy.c

This patch renames BitMask and dwBitMask variables into
bitmask to remove camel case.
dwBitMask in the given context can be renamed into simply
bitmask since its number of bits (dw->32) can be deduced
by its type which is u32.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>