mtd: rawnand: Remove of_get_nand_on_flash_bbt() wrapper

Remove the wrapper as it hides for no reason what we really want: find an
of_property. Removing the wrapper makes the code easier to read.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220106131610.225661-1-tudor.ambarus@microchip.com