mos7720 update

this driver has an interesting way of handling ENOMEM: complain and ignore.
If you decide to live with allocation failures, you must

1. guard against URBs without corresponding buffers
2. complete allocation failures
3. always test entries for NULL before you follow the pointers

This patch does so.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>