nvme: constify variable in list_subsys

nvme: constify variable in list_subsys

The compiler complains with

../nvme.c: In function ‘list_subsys’:
../nvme.c:2478:19: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
 2478 |                 n = strrchr(devicename, 'n');
      |                   ^

devicename is of type 'const char *' so the returning pointer is of
the same type. Thus constify the n variable.

Signed-off-by: Daniel Wagner <dwagner@suse.de>