mtd: nand: complete nand_register() arguments check

The patch checks that the "mtd" parameter is accessible before
proceeding.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>
Link: https://lore.kernel.org/all/20231102112743.57420-1-dario.binacchi@amarulasolutions.com