If the i2c write fails during initialisation, an oops happens
because state->frontend.dvb is still undefined. Fixed.

Thanks to Sigmund Augdal for reporting this bug,
and to Hartmut Birr for suggesting the fix.

Thanks-to: Sigmund Augdal <sigmund@snap.tv>
Thanks-to: Hartmut Birr <e9hack@gmail.com>
Signed-off-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>