media: mxl5xx: fix tuning logic

The tuning logic is broken with regards to status report:
it relies on a previously-cached value that may not be valid
if re-tuned.

Change the logic to always read the status.

Acked-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>