mtd: nand: Rename nand_get_flash_type() into nand_detect()

Since commit 4722c0e9 ("mtd: nand: change return type of
nand_get_flash_type() to int"), nand_get_flash_type() no longer returns
a nand_flash_dev object.
Rename the function to match this new behavior.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Richard Weinberger <richard@nod.at>