media: stk1160: Simplify the build config definition

Commit dfb9f94e ("[media] stk1160: Build as a module if SND is m and
audio support is selected") had to introduce some complex config dependency
handling to compile for all combinations of configs VIDEO_STK1160 and
VIDEO_STK1160_AC97.

Later, commit e36e6b5f ("[media] stk1160: Remove stk1160-mixer and
setup internal AC97 codec automatically") removes the config
VIDEO_STK1160_AC97, which renders the previous dependency handling
unnecessary. The commit already simplified the dependency of the remaining
config VIDEO_STK1160, but it misses the opportunity to merge VIDEO_STK1160
and VIDEO_STK1160_COMMON.

So, do that now and simplify the build config definition of the STK1160 USB
video capture support.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>