unix_serial_passthrough.c: check errno for EWOULDBLOCK

plat_serpt_write_vcon(): write() returns how much data it has written
to the file descriptor, and in case an error, it returns -1. So the
EWOULDBLOCK never really triggers, as in the following condition we're
not checking the errno, but the return value of the write() function.

src/unix/unix_serial_passthrough.c

@@ -112,7 +112,7 @@ plat_serpt_write_vcon(serial_passthrough_t *dev, uint8_t data)
     if (dev->mode == SERPT_MODE_HOSTSER) {
         do {
             res = write(dev->master_fd, &data, 1);
-        } while (res == 0 || (res == -1 && (errno == EAGAIN || res == EWOULDBLOCK)));
+        } while (res == 0 || (res == -1 && (errno == EAGAIN || errno == EWOULDBLOCK)));
     } else
         res = write(dev->master_fd, &data, 1);
 }