[media] v4l2-ctrls: rewrite copy routines to operate on union v4l2_ctrl_ptr

In order to implement array support and (for the future) configuration stores
we need to have more generic copy routines that all operate on the v4l2_ctrl_ptr
union. So instead of e.g. using ctrl->cur.string it uses ptr.p_char. This makes
e.g. cur_to_user generic so it can be used to copy any v4l2_ctrl_ptr value to
userspace, not just the (hardcoded) current value.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>