mtd: nand: add a helper to detect the nand type

This helper detects that whether the mtd's type is nand type.

Now, it's clear that the MTD_NANDFLASH stands for SLC nand only.
So use the mtd_type_is_nand() to replace the old check method
to do the nand type (include the SLC and MLC) check.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

drivers/mtd/tests/oobtest.c

@@ -289,7 +289,7 @@ static int __init mtd_oobtest_init(void)
 		return err;
 	}
 
-	if (mtd->type != MTD_NANDFLASH) {
+	if (!mtd_type_is_nand(mtd)) {
 		pr_info("this test requires NAND flash\n");
 		goto out;
 	}