[media] cinergyT2-fe: cache stats at cinergyt2_fe_read_status()

Instead of sending USB commands for every stats call, collect
them once, when status is updated. As the frontend kthread
will call it on every few seconds, the stats will still be
collected.

Besides reducing the amount of USB/I2C transfers, this also
warrants that all stats will be collected at the same time,
and makes easier to convert it to DVBv5 stats in the future.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>