media: b2c2-flexcop-usb: add sanity checking

The driver needs an isochronous endpoint to be present. It will
oops in its absence. Add checking for it.

Reported-by:  <syzbot+d93dff37e6a89431c158@syzkaller.appspotmail.com>
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>