media: cx231xx: replace BUG_ON with recovery code

In cx231xx_i2c_register, if dev->cx231xx_send_usb_command is NULL,
the code crashes. However, the callers in cx231xx-core are able to
handle the error without crashing. This patch fixes this issue.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>