drivers/ddr/fsl: fix '__hwconfig without a buffer' messages

Pass an empty buffer instead of NULL if the hwconfig environment
variable isn't set.

Signed-off-by: Jeremy Gebben <jgebben@sweptlaser.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: York Sun <york.sun@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>