media: sun6i-isp: proc: Error out on invalid port to fix warning

The enabled variable is only set for a valid port and used later,
which triggers an uninitialized use smatch warning. Explicitly error
out in that case to fix the warning.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Fixes: e3185e1d ("media: staging: media: Add support for the Allwinner A31 ISP")
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>