mtd: rawnand: Inline onfi_get_async_timing_mode()

onfi_get_async_timing_mode() is only used in one place inside
nand_base.c. Let's inline the code and kill the helper.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>