V4L/DVB (7348): tuner-simple: do not send i2c commands if there is no i2c adapter

if (priv->i2c_props.adap == NULL) then exit any function that would
send commands over the i2c bus.  We allow drivers to attach without an
i2c adapter for cases where the dvb demod accesses the tuner directly
via calc_regs.

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