V4L/DVB (10219): saa7134: Prevent Oops due to stale IRQ status when enabling interrupts

When enabling a shared IRQ line, then saa7134_irq handler could be invoked
before the driver had completely set up internal structures, due to a shared
interrupt line firing.  Clear the saa7134 interrupt status reg, before
requesting the irq line, so that stale IRQ status isn't processed before the
internal structures are set up.

Marcin Slusarz recently brought this Oops to the attention of the v4l-dvb
lists and provided an initial analysis by investigating reports found here:

http://kerneloops.org/guilty.php?guilty=mute_input_7133&version=2.6.27-release&start=1802240&end=1835007&class=oops



Reported-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>