mtd: rawnand: sunxi: Add an U suffix to NFC_PAGE_OP definition

Fixes the "warning: large integer implicitly truncated to unsigned type
[-Woverflow]" warning when compiled for x86.

This is needed in order to allow compiling this driver when
COMPILE_TEST=y.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>