staging: rtl8723bs: Move bandwidth index lookup to new function.

Factoring out the conditional lookup of bandwidth index into the power
limit table into it's own function simplifies the logic of
'phy_get_tx_pwr_lmt()'.

Signed-off-by: Quytelda Kahja <quytelda@tamalin.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>