[media] v4l: vsp1: Propagate vsp1_device_get errors to the callers

Modify the vsp1_device_get() function to return an error code instead of
a pointer to the VSP1 device, and use the return value in the callers.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>