media: i2c: tc358746: check fmt validity

Check if the format was really found.

Fixes smatch warning:

drivers/media/i2c/tc358746.c:790 tc358746_set_fmt() error: 'fmt' dereferencing possible ERR_PTR()

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
CC: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>