b43: Fix oops on unload when firmware not found

When b43 fails to find firmware when loaded, a subsequent unload will
oops due to calling ieee80211_unregister_hw() when the corresponding
register call was never made.

Commit 2d838bb6 fixed the same problem
for b43legacy.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Tested-by: Markus Kanet <dvmailing@gmx.eu>
Cc: Stable <stable@vger.kernel.org> [V3.3.0+ (the patch will need to be refactored)]
Cc: Markus Kanet <dvmailing@gmx.eu>
Signed-off-by: John W. Linville <linville@tuxdriver.com>