mtd: adapt misleading comment in mtd_oob_ops structure

A comment in the kernel doc of the mtd_oob_ops structure tells that it
is not possible to write more than one page with OOB. This is actually
true for only a few MTD devices like 'onenand' but it is definitely not
a general limitation. While this would benefit to be handled elsewhere
either by the MTD layer or by the limited drivers, let's update this
comment to reflect the reality.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>