tda1997x: get rid of an unused var

1 warning regressions:
  + drivers/media/i2c/tda1997x.c: warning: variable 'last_irq_status' set but not used [-Wunused-but-set-variable]:  => 1421:17

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