PPC4xx: ALPR: constify FPGA code

The ALPR custom FPGA code was missed by commit e6a857d "fpga:
constify to fix build warning" resulting in such warnings:

fpga.c:226: warning: initialization from incompatible pointer type

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>
Acked-by: Stefan Roese <sr@denx.de>

board/prodrive/alpr/fpga.c

@@ -168,7 +168,7 @@ int fpga_done_fn (int cookie)
 /* writes the complete buffer to the FPGA
    writing the complete buffer in one function is much faster,
    then calling it for every bit */
-int fpga_write_fn (void *buf, size_t len, int flush, int cookie)
+int fpga_write_fn (const void *buf, size_t len, int flush, int cookie)
 {
 	size_t bytecount = 0;
 	unsigned char *data = (unsigned char *) buf;