mtd: remove stale 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 was
probably true at some time in the past but today it is entirely wrong.

As one can see for instance in the nand_do_write_ops() helper available
in the NAND core, this implementation called by mtd->_write_oob()
simply loops over the pages until everything has been written.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>