staging: vt6655: use workqueue for interrupt handling

Introduce vnt_interrupt to handle interrupt and use workqueue
to queue and queue on vif.

Convert device_intr to void call vnt_interrupt_process
from vnt_interrupt_work providing vif is valid.

This removes troublesome heavy code from the interupt handler and
allows to remove atomic from other areas of driver.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>