[media] v4l2: Fix the type of V4L2_CID_TUNE_PREEMPHASIS in the documentation

Change the type of V4L2_CID_TUNE_PREEMPHASIS from 'integer' to 'enum
v4l2_preemphasis'

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>