staging: vt6655: Fix interrupt race condition on device start up.

It appears on some slower systems that the driver can find its way
out of the workqueue while the interrupt is disabled by continuous polling
by it.

Move MACvIntEnable to vnt_interrupt_work so that it is always enabled
on all routes out of vnt_interrupt_process.

Move MACvIntDisable so that the device doesn't keep polling the system
while the workqueue is being processed.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
CC: stable@vger.kernel.org # v4.2+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>