V4L/DVB (11452): gspca - m5602-po1030: Convert to have a v4l2 ctrl cache

Let the po1030 have a local v4l2 ctrl cache as this minimizes the load on reading the registers and improves performance.

Signed-off-by: Erik Andr?n <erik.andren@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>