mtd: Initialize all parameters of mtd_oob_ops

Most of the time the ooboffs parameter of the mtd_oob_ops structure
was initialized only when needed. Since the introduction of the
SPI-NAND subsystem, this parameter is transferred into
nand_page_io_req structure automatically and may be used by any
SPI-NAND user.

Before this happens, initialize all the structure parameters when they
are created in mtdchar.c.

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