media: i2c: Fix max gain in ov8865

The maximum gain figure in the v4l2 ctrl is wrong. The field is 12 bits
wide, which is where the 8191 figure comes from, but the datasheet is
specific that maximum gain is 16x (the low seven bits are fractional, so
16x gain is 2048)

Signed-off-by: Daniel Scally <djrscally@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>