staging: rtl8723bs: Move rate section index lookup to new function.

The rate section lookup is a large switch statement in the middle of
'phy_get_tx_pwr_lmt()'; refactor this statement into it's own function
for increased readability.

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