V4L/DVB (5295): Digitv: open nxt6000 i2c_gate for TDED4 tuner handling

dvb-pll normally opens the i2c gate before attempting to communicate with
the pll, but the code for this device is not using dvb-pll.  This should
be cleaned up in the future, but for now, just open the i2c gate at the
appropriate place in order to fix this driver bug.

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