mac80211: Fix compilation warning when CONFIG_INET is not set

The warning is:
  net/mac80211/main.c:688: warning: label ‘fail_ifa’ defined but not used

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>