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.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Florian Schilhabel <florian.c.schilhabel@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>