fpga: constify to fix build warning

Fix compiler warning:

cmd_fpga.c:318: warning: passing argument 3 of 'fit_image_get_data'
from incompatible pointer type

Adding the needed 'const' here entails a whole bunch of additonal
changes all over the FPGA code.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Andre Schwarz <andre.schwarz@matrix-vision.de>
Cc: Murray Jensen <Murray.Jensen@csiro.au>
Acked-by: Andre Schwarz<andre.schwarz@matrix-vision.de>
diff --git a/board/matrix_vision/mergerbox/fpga.c b/board/matrix_vision/mergerbox/fpga.c
index 673bc2c..021e9c4 100644
--- a/board/matrix_vision/mergerbox/fpga.c
+++ b/board/matrix_vision/mergerbox/fpga.c
@@ -161,7 +161,7 @@
 	return 0;
 }
 
-int fpga_wr_fn(void *buf, size_t len, int flush, int cookie)
+int fpga_wr_fn(const void *buf, size_t len, int flush, int cookie)
 {
 	unsigned char *data = (unsigned char *) buf;
 	int i;