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>

drivers/fpga/zynqpl.c

@@ -421,7 +421,8 @@ static int zynq_loadfs(xilinx_desc *desc, const void *buf, size_t bsize,
 	loff_t blocksize, actread;
 	loff_t pos = 0;
 	int fstype;
-	char *interface, *dev_part, *filename;
+	char *interface, *dev_part;
+	const char *filename;
 
 	blocksize = fsinfo->blocksize;
 	interface = fsinfo->interface;