Fix buggy /dev/nvme matching in the case statement to determine...

Fix buggy /dev/nvme matching in the case statement to determine disc_offered_devfs (Closes: #799119).

Thanks, Mario Limonciello!

Because of course the patterns one uses in sed substitutions aren't
exactly the same one wants to be using in case statements.

This fixes the regression introduced in:
| commit 7046795c
| Author: Cyril Brulebois <kibi@debian.org>
| Date:   Tue May 12 22:38:32 2015 +0200
|
|     Add support for multi-digit X and Y in /dev/nvmeXnY