mtd: rawnand: omap: Rename a macro

The macro BADBLOCK_MARKER_LENGTH is pretty long and could be reduced to
BBM_LEN which is more handy to use in the code.

This is a purely cosmetic change and is only done to avoid further
change to contain 100+ char lines just because of this definition.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210610134906.3503303-3-miquel.raynal@bootlin.com