[media] dvb-bt8xx: handle errors from dvb_net_init

Clean up and error out if dvb_net_init fails (for example when
running out of memory).

From an audit of dvb_net_init callers, now that dvb_net_init
has learned to return a nonzero value from time to time.

[mchehab.redhat.com: codingstyle fix: printk() should include KERN_ facility level]
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>