devsd: fix sddelpart() AGAIN

the previous "fix" missed to advance pp so after
we hit a invalid partition all following partitions
would be ignored.

sys/src/9/port/devsd.c

@@ -154,9 +154,7 @@ sddelpart(SDunit* unit, char* name)
 	 */
 	pp = unit->part;
 	for(i = 0; i < unit->npart; i++){
-		if(!pp->valid)
-			continue;
-		if(strcmp(name, pp->name) == 0)
+		if(pp->valid && strcmp(name, pp->name) == 0)
 			break;
 		pp++;
 	}