mtd: cfi_cmdset_0002: Change write buffer to check correct value

For the word write it is checked if the chip has the correct value.
But it is not checked for the write buffer as only checked if ready.
To make sure for the write buffer change to check the value.

It is enough as this patch is only checking the last written word.
Since it is described by data sheets to check the operation status.

Signed-off-by: Tokunori Ikegami <ikegami@allied-telesis.co.jp>
Reviewed-by: Joakim Tjernlund <Joakim.Tjernlund@infinera.com>
Cc: Chris Packham <chris.packham@alliedtelesis.co.nz>
Cc: Brian Norris <computersforpeace@gmail.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>
Cc: linux-mtd@lists.infradead.org
Cc: stable@vger.kernel.org
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>