V4L/DVB (7184): make hybrid_tuner_request_state tolerant of devices without i2c adapters

Some dvb demodulators access the tuner directly without using i2c.
In these cases, i2c_adap may be NULL.  This patch fixes
hybrid_tuner_request_state to allow for NULL i2c_adapters.

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