rtlwifi: fix for race condition when firmware is cached

In commit b0302ab, 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>