[media] v4l2-ctrls: return elem_size instead of strlen

When getting a string and the size given by the application is too
short return the max length the string can have (elem_size) instead
of the string length + 1. That makes more sense.

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