staging: wilc1000: Destroy mutex object in deinitialization

Destroy the mutex object that is initialized in wlan_init_locks()

Signed-off-by: HariPrasath Elango <hariprasath.elango@gmail.com>
Reviewed-by: Ajay Singh <ajay.kathat@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>