spl: fix descending condition to drivers/mtd/nand/

SPL should not reference CONFIG_CMD_NAND to decide whether or not
it should build drivers/mtd/nand.  CONFIG_CMD_NAND should be only
used to select the NAND utility command on the command parser.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>