[media] em28xx-dvb: don't initialize drx-d non-used fields with zero

There's no need to initialize unused fields with zero, as Kernel does
it automatically.

Removing the initialization makes the code cleaner.

This also allows the removal of the unused pll_set callback.

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