mtd: spinand: Handle the case where PROGRAM LOAD does not reset the cache

Looks like PROGRAM LOAD (AKA write cache) does not necessarily reset
the cache content to 0xFF (depends on vendor implementation), so we
must fill the page cache entirely even if we only want to program the
data portion of the page, otherwise we might corrupt the BBM or user
data previously programmed in OOB area.

Fixes: 7529df46 ("mtd: nand: Add core infrastructure to support SPI NANDs")
Reported-by: Stefan Roese <sr@denx.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Tested-by: Stefan Roese <sr@denx.de>
Reviewed-by: Stefan Roese <sr@denx.de>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>