[media] use v4l2_get_timestamp where possible

This is a preparation for a change to the type of v4l2 timestamps.
v4l2_get_timestamp() is a helper function that reads the monotonic
time and stores it into a 'struct timeval'. Multiple drivers implement
the same thing themselves for historic reasons.

Changing them all to use v4l2_get_timestamp() is more consistent
and reduces the amount of code duplication, and most importantly
simplifies the following changes.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
[hans.verkuil@cisco.com: dropped the v4l2-dev.c patch that didn't belong here]
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>