rtlwifi: fix for race condition when firmware is cached

In commit b0302aba

, the rtlwifi family of drivers was converted to use
asynchronous firmware loading. Unfortumately, the implementation was
racy, and the ieee80211 routines could be started before rtl_init_core()
was called to setup the data.

This patch fixes the bug noted in https://bugzilla.kernel.org/show_bug.cgi?id=43187.

Reported-by: Joshua Roys <Joshua.Roys@gtri.gatech.edu>
Tested-by: Neptune Ning <frostyplanet@gmail.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org>    [3.3]
Signed-off-by: John W. Linville <linville@tuxdriver.com>