mtd: rawnand: Propagate error and simplify ternary operators for brcmstb_nand_wait_for_completion()

As bcmnand_ctrl_poll_status() return negative errno, so return true if
sts < 0. The < 0 case does not exist for wait_for_completion_timeout(),
so return true if sts = 0 and zero otherwise. Both of the true return
of them can be considered as a -ETIMEDOUT err, so return -ETIMEDOUT
if err is true to propagate err from its caller.

Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Reviewed-by: William Zhang <william.zhang@broadcom.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230808032943.3890545-1-ruanjinjie@huawei.com