V4L/DVB (6668): Fix theoretical races between IRQ handler and .suspend/resume

*dev->insuspend = 1 should be set before synchronize_irq
 *ACK interrupts after synchronize_irq, to make sure there aren't
 pending interrupts.
 *Add barrier before we restart interrupts so the handler will 100%
 see the dev->insuspend

Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>