mtd: rawnand: Fix nand_gpio_waitrdy()

Mimic what's done in nand_soft_waitrdy() and add one to the jiffies
timeout so we don't end up waiting less than actually required.

Reported-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Fixes: b0e137ad

 ("mtd: rawnand: Provide helper for polling GPIO R/B pin")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200518155237.297549-1-boris.brezillon@collabora.com