staging: vt6656: [bug] Fix missing spin lock in iwctl_siwpower.

Fixes occasional dead lock on power up / down.

spin_lock_irq is used because of unlocking with spin_unlock_irq
elsewhere in the driver.

Only relevant to kernels 3.8 and later when command was
transferred to the iw_handler.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Cc: stable@vger.kernel.org # 3.8+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>