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 <[email protected]>

nvme.c

@@ -2473,7 +2473,8 @@ static int list_subsys(int argc, char **argv, struct command *cmd,
 	}
 
 	if (devicename) {
-		char *n, *eptr = NULL;
+		const char *n;
+		char *eptr = NULL;
 
 		n = strrchr(devicename, 'n');
 		if (!n) {