AP_HAL_PX4: UARTDriver: fix writting with failures

When the buffer wraps and we do it in 2 steps, we can't actually do the
second part if it fails or if we wrote less bytes than we intended,
otherwise we will corrupt the data being sent.

While at it, just like in the write case, explain why we are stopping.

libraries/AP_HAL_PX4/UARTDriver.cpp

@@ -445,8 +445,15 @@ void PX4UARTDriver::_timer_tick(void)
         const auto n_vec = _writebuf.peekiovec(vec, n);
         for (int i = 0; i < n_vec; i++) {
             ret = _write_fd(vec[i].data, (uint16_t)vec[i].len);
-            if (ret > 0)
-                _writebuf.advance(ret);
+            if (ret < 0) {
+                break;
+            }
+            _writebuf.advance(ret);
+
+            /* We wrote less than we asked for, stop */
+            if ((unsigned)ret != vec[i].len) {
+                break;
+            }
         }
         perf_end(_perf_uart);
     }
@@ -458,11 +465,15 @@ void PX4UARTDriver::_timer_tick(void)
     const auto n_vec = _readbuf.reserve(vec, _readbuf.space());
     for (int i = 0; i < n_vec; i++) {
         ret = _read_fd(vec[i].data, vec[i].len);
-        if (ret < 0)
+        if (ret < 0) {
             break;
+        }
         _readbuf.commit((unsigned)ret);
-        if ((unsigned)ret < vec[i].len)
+
+        /* stop reading as we read less than we asked for */
+        if ((unsigned)ret < vec[i].len) {
             break;
+        }
     }
     perf_end(_perf_uart);