mtd: nand: drop EXPORT_SYMBOL_GPL for nanddev_erase()

This function is only used within this module, so it is no longer
necessary to use EXPORT_SYMBOL_GPL().

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20221018170205.1733958-1-dario.binacchi@amarulasolutions.com