staging: rtl8723bs: hal: Replace function PHY_GetTxPowerIndex()

Remove function PHY_GetTxPowerIndex as it does nothing except call
PHY_GetTxPowerIndex_8723B.
Rename PHY_GetTxPowerIndex_8723B to PHY_GetTxPowerIndex to maintain
compatibility with call site.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Link: https://lore.kernel.org/r/20190725052645.2372-3-nishkadg.linux@gmail.com


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