fpga: Replace char * with const char * for filename

Ensure the string for filename is always constant, otherwise it can be
corrupted by the writing.

Signed-off-by: Tien Fong Chee <tien.fong.chee@intel.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

include/fpga.h

@@ -41,7 +41,7 @@ typedef struct {                /* typedef fpga_desc */
 	unsigned int blocksize;
 	char *interface;
 	char *dev_part;
-	char *filename;
+	const char *filename;
 	int fstype;
 } fpga_fs_info;