media: staging: atomisp: reenable warnings for I2C

When atomisp got merged, there were so many warnings with W=1
that we simply disabled the ones that were causing troubles.

Since then, several changes got applied to atomisp, and the
number of warnings are a way smaller than it used to be.

So, let's reenable warnings there and fix the issues.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>