media: em28xx: use a default format if TRY_FMT fails

Follow the V4L2 spec, as warned by v4l2-compliance:

	warn: v4l2-test-formats.cpp(732): TRY_FMT cannot handle an invalid pixelformat.
	warn: v4l2-test-formats.cpp(733): This may or may not be a problem. For more information see:

warn: v4l2-test-formats.cpp(734): http://www.mail-archive.com/linux-media@vger.kernel.org/msg56550.html



Cc: stable@vger.kernel.org
Fixes: bddcf633 ("V4L/DVB (9927): em28xx: use a more standard way to specify video formats")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>