mtd: rawnand: Use unsigned types for nand_chip unsigned values

page_shift, phys_erase_shift, bbt_erase_shift, chip_shift, pagemask,
subpagesize and badblockbits are all positive values, so declare
them as unsigned.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200529111322.7184-2-miquel.raynal@bootlin.com