staging: rtl8712, rtl8712: avoid lots of build warnings
The rtl8712 driver has an 'extern inline' function that contains an
'if', which causes lots of warnings with CONFIG_PROFILE_ALL_BRANCHES
overriding the definition of 'if':
drivers/staging/rtl8712/ieee80211.h:759:229: warning: '______f' is static but declared in inline function 'ieee80211_get_hdrlen' which is not static [enabled by default]
This changes the driver to use 'static inline' instead, which happens
to be the correct annotation anyway.