FPGA: constify Lattice FPGA code

The Lattice code was missed by commit e6a857d "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>
diff --git a/include/lattice.h b/include/lattice.h
index e965663..6a2cf93 100755
--- a/include/lattice.h
+++ b/include/lattice.h
@@ -302,8 +302,8 @@
 void ispVMDelay(unsigned short int a_usMicroSecondDelay);
 void calibration(void);
 
-int lattice_load(Lattice_desc *desc, void *buf, size_t bsize);
-int lattice_dump(Lattice_desc *desc, void *buf, size_t bsize);
+int lattice_load(Lattice_desc *desc, const void *buf, size_t bsize);
+int lattice_dump(Lattice_desc *desc, const void *buf, size_t bsize);
 int lattice_info(Lattice_desc *desc);
 
 void ispVMStart(void);