xerox_mfp: check the list of malfunctioning devices elsewhere, make checks less strict

The str*() functions are called several times during a single image transfer. If devices inappropriately advertise JPEG support, it is better to disable the support onces in dev_inqiry()

There are pieces of SCX-4500W that don't have their model string begin with the model name. Therefore, strncmp() doesn't detect these devices properly. Use strstr() instead.