serial: get rid of overly verbose spew

It just made spew-level logging unusable; the way sigrok async comms work
guarantees most of it was a false error.

hardware/common/serial.c

@@ -278,13 +278,7 @@ SR_PRIV int serial_read(struct sr_serial_dev_inst *serial, void *buf,
 #else
 	/* Returns the number of bytes read, or -1 upon failure. */
 	ret = read(serial->fd, buf, count);
-	if (ret < 0)
-		/*
- 		 * Should be sr_err(), but that would yield lots of
-		 * "Resource temporarily unavailable" messages.
-		 */
-		sr_spew("Read error: %s (fd %d).", strerror(errno), serial->fd);
-	else
+	if (ret >= 0)
 		sr_spew("Read %d/%d bytes (fd %d).", ret, count, serial->fd);
 #endif