media: i2c: alvium: rename acquisition frame rate enable reg

Acquisition frame rate enable reg have a very long name let's reduce this
with an abbreviation.

Signed-off-by: Tommaso Merciai <tomm.merciai@gmail.com>
[Sakari Ailus: Fix spelling of "acquisition".]
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>