For a number of different ioctls, the v4l2-ioctl code checks that the
passed buffer type is supported by the driver.  It did this by checking
that the driver defined a method for the try_fmt handler for that buffer
type.  However, try_fmt is optional and a driver might not provide it even
though it does support that type.  So use g_fmt instead, since that isn't
optional.

This should fix a problem with VBI capture with saa7146.
Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>