[media] v4l2-tpg: Init hv_enc field with a valid value

Zero is not a valid value for hsv_enc. Set the field to a valid
initial value.

This is not a problem for vivid, because it sets the field to 180 via
tpg_s_hsv_enc() on the control initialization, but it might be a source
of errors for other drivers that use this code.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>