V4L/DVB (3268): Move video std detection to top of set_tv_freq function

- move video std detection to top of set_tv_freq function
- we must detect video std first, so that we can choose the correct
  tuner_params

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>