mtd: nand: supress 'unknown NAND' warning if no nand is found

This printk was added recently and results in ugly output on systems
with no NAND:

NAND:  nand_get_flash_type: unknown NAND device: Manufacturer ID: 0x00, Chip ID: 0x00 0 MiB

instead of:

NAND:  0 MiB

Signed-off-by: Steve Sakoman <steve@sakoman.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>