mtd: nand: check nand_mtd_to_devnum() argument

If the "mtd" parameter is NULL, the search will definitely yield a
negative result. In that case, it's better to exit immediately.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>
Link: https://lore.kernel.org/all/20231102113829.58852-1-dario.binacchi@amarulasolutions.com