V4L/DVB (5987): saa7146: clean-up irq processing

Interrupt processing fixed: First handle interrupt, then acknowledge it.
Otherwise the same interrupt might occur twice.
Cleaned-up i2c interrupt handler and i2c error messages.

Signed-off-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>