staging: r8188eu: remove unused PHY_UpdateTxPowerDbm8188E()

Function PHY_UpdateTxPowerDbm8188E() is unused and it is the only user
of function phy_DbmToTxPwrIdx(), remove both.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20210908223903.10516-4-straube.linux@gmail.com


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