uboot-imx

cmd: spl: fix compiling error when CONFIG_CMD_SPL_WRITE_SIZE not defined

CONFIG_CMD_SPL_WRITE_SIZE is used for writing parameters to non-volatile
storage. So far it is only used for NAND. Fix compiling error when this
macro is not used for SD.

Signed-off-by: York Sun <york.sun@nxp.com>
CC: Anatolij Gustschin <agust@denx.de>