While the driver has support for both serial and parallel mode,
There's was way to select serial mode via configuration. Add
a config option for that, while keeping the default in serial mode.

Also, at debug mode, it will now print a message when mpeg is
enabled/disabled, and showing if parallel or serial mode were
selected, helping developers to double-check if the DRX-K is at
the right mode.
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>