staging: wilc1000: free allocated memory in wilc_add_rx_gtk()

Free memory allocated in wilc_add_rx_gtk() before returing from the
function.

Signed-off-by: Ajay Singh <ajay.kathat@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>