device-dax: fix debug output typo

The debug output for return the return data of pgoff_to_phys() in the
fault handlers has 'phys' and 'pgoff' incorrectly swapped.

Reported-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>