at91, smartweb: use SPL_SYS_MALLOC_F_LEN

commit f1896c45: spl: make SPL and normal u-boot stage use independent SYS_MALLOC_F_LEN
introduced independent SYS_MALLOC_F_LEN for SPL and U-Boot.

Use it on the smartweb board, as above commit broke
the smartweb board.

Signed-off-by: Heiko Schocher <hs@denx.de>