media: uvcvideo: Set the colorspace as sRGB if undefined

Never return V4L2_COLORSPACE_DEFAULT. From the standard:

"""
In the absence of this descriptor, or in the case of
“Unspecified” values within the descriptor, color matching
defaults will be assumed. The color matching defaults are
compliant with sRGB since the BT.709 transfer function and
the sRGB transfer function are very similar.
"""

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Tested-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>