mtd: rawnand: Fix return value check of wait_for_completion_timeout

wait_for_completion_timeout() returns unsigned long not int.
It returns 0 if timed out, and positive if completed.
The check for <= 0 is ambiguous and should be == 0 here
indicating timeout which is the only error case.

Fixes: 83738d87 ("mtd: sh_flctl: Add DMA capabilty")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220412083435.29254-1-linmq006@gmail.com