staging: r8188eu: make LPS_RF_ON_check static

LPS_RF_ON_check is used only in rtw_pwrctrl.c. Make it a static function.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220409163212.241122-7-martin@kaiser.cx


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