cam/mmc: Use xpt_path_inq instead of hand-rollded equivalent

There's no reason to inline xpt_path_inq here. While there was a
mismatch between this use (CAM_PRIORITY_NONE) and the old xpt_path_inq
code (which used CAM_PRIORITY_NORMAL), xpt_path_inq has been corrected
to use _NONE since this is a non-queued command.

Sponsored by: Netflix
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D51168