rtlwifi: Use ffs in <foo>_phy_calculate_bit_shift

Remove the loop and use the generic ffs instead.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/e2ab424d24b74901bc0c39f0c60f75e871adf2ba.camel@perches.com