mtd: uboot: Fix hanging during mtd list command

mtd: uboot: Fix hanging during mtd list command

Some boards (like omap3_logic) hang when trying to access
address 0. This happens when executing the new 'mtd list' command.
This patch enhances the checks for conditions that would
preclude mtd_probe_devices() from operating.

Fixes: 3a11299ea117 ("cmd: mtd: add 'mtd' command")
Suggested-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>