media: v4l: ioctl: Allow drivers to fill in the format description

The v4l_fill_fmtdesc() function does a good job in filling in pixelformat
description. While generally all drivers should depend on this function
doing the job, staging drivers that use their own formats may not.

Allow staging drivers to fill in their own formats by checking whether the
description begins with a non-nil character before issuing the warning.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Suggested-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>