staging: wilc1000: fix memory leak in wilc_add_rx_gtk

In case *mode* happens to be different than WILC_AP_MODE and
WILC_STATION_MODE, gtk_key is not released, hence leanding
to a memory leak. So, in this case it is safer to release
gtk_key just before returning to callers.

Addresses-Coverity-ID: 1476020 ("Resource leak")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>