staging: rtl8723au: Do not write garbage to REG_SPS0_CTRL

Use a separate variable to hold the value of REG_SPS0_CTRL to avoid it
getting clobbered and writing out garbage to it, for the case of
eRFPowerState == rf_sleep/rf_off && bRegSSPwrLvl == 1.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>