[media] tuner-core: fix s_std and s_tuner

Both s_std and s_tuner are broken because set_mode_freq is called before the
new std (for s_std) and audmode (for s_tuner) are set.

This patch splits set_mode_freq in a set_mode and a set_freq and in s_std/s_tuner
first calls set_mode, and if that returns 0 (i.e. the mode is supported)
then they set t->std/t->audmode and call set_freq.

This fixes a bug where changing std or audmode would actually change it to
the previous value.

Discovered while testing analog TV standards for cx18 with a tda18271 tuner.

Cc: stable@kernel.org
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>