nand: Fix some more NULL name tests

Now that nand_info[] is an array of pointers we need to test the
pointer itself rather than using name as a proxy for NULLness.

Fixes: b616d9b0 ("nand: Embed mtd_info in struct nand_chip")
Signed-off-by: Scott Wood <oss@buserror.net>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Tony Lindgren <tony@atomide.com>
Acked-by: Tony Lindgren <tony@atomide.com>