V4L/DVB: saa7134: Fix IRQ2 bit names for the register map

There's an error at the IRQ2 bit map registers. Also, it doesn't
show what bits are needed for positive and for negative edge.

In the case of IR raw decoding, for some protocols, it is important
to detect both positive and negative edges. So, a latter patch
will need to use the other values.

Also, the code that detects problems on IRQ handling is incomplete,
as it disables only one of the IRQ bits for GPIO16 and GPIO18.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>