V4L/DVB (10058): dsbr100: fix codingstyle, add dev_err messages

We should make if-constructions more clear. Introduce int retval
variables in some functions to make it this way. Also a lot of useful
dev_err messages added. And now we check every returned value after
usb_control_msg.

Signed-off-by: Alexey Klimov <klimov.linux@gmail.com>
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>