An error was returned if composing was not supported, instead of if
cropping was not supported.

A classic copy-and-paste bug. Found with v4l2-compliance.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: <stable@vger.kernel.org>      # for v3.18
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>