media: dvb_frontend: ensure that the step is ok for both FE and tuner

The frequency step should take into account the tuner step,
as, if tuner step is bigger than frontend step, the zigzag
algorithm won't be doing the right thing, as it will be
tuning multiple times at the same frequency.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>