mtd: nand: add a helper to check the SLC/MLC nand chip

Add a helper to check if a nand chip is SLC or MLC.
This helper makes the code more readable.

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

diff --cc drivers/mtd/nand/denali.c
Simple merge

diff --cc drivers/mtd/nand/nand_base.c
Simple merge

diff --cc include/linux/mtd/nand.h
Simple merge