xerox_mfp: return correct value in dev_acquire()

dev_acquire() is expected to return 1 on success and 0 on failure. Fix the cases where it returned non-zero on error.

The second block changes ret_cancel(dev, SANE_STATUS_NO_MEM); to ret_cancel(dev, SANE_STATUS_NO_MEM);\n return 0;. The alternative would be to just return ret_cancel(dev, 0); - I figure that would hide information from ret_cancel() that might be useful when debugging some future error, but I'm happy to resubmit that way if it would be better.