FPGA: constify Lattice FPGA code

The Lattice code was missed by commit e6a857da "fpga: constify to fix
build warning" resulting in such warnings:

fpga.c: In function 'fpga_load':
fpga.c:238: warning: passing argument 2 of 'lattice_load' discards qualifiers from pointer target type
fpga.c: In function 'fpga_dump':
fpga.c:278: warning: passing argument 2 of 'lattice_dump' discards qualifiers from pointer target type

Signed-off-by: Wolfgang Denk <wd@denx.de>
cc: Stefano Babic <sbabic@denx.de>