staging: r8188eu: use ffs() in phy_CalculateBitShift()

Use ffs() in phy_CalculateBitShift() to simplify the function and
improve readability.

Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20220806060929.11022-2-straube.linux@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>