brymen-dmm: Make serial write call block.

This call is executed from an event handler and was previously nonblocking,
but has no partial write handling. It sends a short packet so should be OK
to block, most likely the output buffer will be empty anyway.

src/hardware/brymen-dmm/parser.c

@@ -78,7 +78,7 @@ static int bm_send_command(uint8_t command, uint8_t arg1, uint8_t arg2,
 	/* TODO: How to compute the checksum? Hardware seems to ignore it. */
 
 	/* Request reading. */
-	written = serial_write(serial, &cmdout, sizeof(cmdout));
+	written = serial_write_blocking(serial, &cmdout, sizeof(cmdout));
 	if (written != sizeof(cmdout))
 		return SR_ERR;