write result not checked in serial drivers

The drivers po_hi_driver_linux_serial.c and po_hi_driver_leon_serial.c do not perform any check that the number of bytes written to the serial line is the number requested.

E.g.:
n = write (po_hi_c_driver_leon_serial_fd_write, &(__po_hi_c_driver_serial_leon_sender_msg.content), __PO_HI_MESSAGES_MAX_SIZE);

But it is never checked if __PO_HI_MESSAGES_MAX_SIZE == n.

In case that the size of the message __PO_HI_MESSAGES_MAX_SIZE is larger than the maximum possible for the serial line, the message is not transmitted correctly and no error is reported. Furthermore, the synchronization with the reader is lost permanently (the reader always reads __PO_HI_MESSAGES_MAX_SIZE bytes, introducing an offset).

All the drivers should be checked in case that the same error appears elsewhere.