sanei_usb: const_cast data buffer

The libusb_bulk_transfer() API caters to both reads and writes and
as a result of that will not take a const pointer.

sanei/sanei_usb.c

@@ -2456,7 +2456,7 @@ sanei_usb_write_bulk (SANE_Int dn, const SANE_Byte * buffer, size_t * size)
 	  int trans_bytes;
 	  ret = libusb_bulk_transfer (devices[dn].lu_handle,
 				      devices[dn].bulk_out_ep,
-				      buffer,
+				      (unsigned char *) buffer,
 				      (int) *size, &trans_bytes,
 				      libusb_timeout);
 	  if (ret < 0)