Move /dev/nvmeXnY handling close to the /dev/mmcblkX one.

Order matters, at least in the device_to_disk case, since we have a
rather general pattern at the end which would only match the /dev/nvme
prefix. Adjust the occurrence in case $prefix as well for consistency.

Reported-by: Steve Rowe <sarowe@gmail.com>
Signed-off-by: Cyril Brulebois <kibi@debian.org>