mtd: nand: change return type of nand_get_flash_type() to int

Upstream linux commit 4722c0e958e636.

The returned "type" is never used in nand_scan_ident() and spl code

Make nand_get_flash_type() simply return an integer value in order
to avoid unnecessary ERR_PTR/PTR_ERR dance.

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Reviewed-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>