staging: rtl8192u: fix bitshift calculation in r819xU_phy.c

This patch uses the ffs linux built-in routine in the
computation of the bitshift to take advantage of
architecture specific instructions that implement this
computation more efficiently.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>