nand: Fix set_dev checks for no device

If we do nand device 0 command in u-boot on a device that has NAND support
enabled but no NAND chip, we can get data abort at least on omaps.

Fix the issue by replacing the check with nand_info[dev] as
suggested by Scott Wood. The check for name existed before because before
the array-to-pointer conversion there was no way to directly test
nand_info[dev] for emptiness.

Signed-off-by: Tony Lindgren <tony@atomide.com>