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

Since commit d1e1f4e4 ("mtd: nand: add support for reading ONFI
parameters from NAND device"), the returned "type" is never used in
nand_scan_ident().

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

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>