V4L/DVB (11568): cx18: Fix the handling of i2c bus registration error

* Return actual error values as returned by the i2c subsystem, rather
  than 0 or 1.
* If the registration of the second bus fails, unregister the first one
  before exiting, otherwise we are leaking resources.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>