Fix UARTSerial emergency unbuffered write

The length calculation in UARTSerial::write_unbuffered was wrong,
meaning it would truncate output data to half length.

This would show up if `platform.stdio-buffered-serial` was configured to
true, `platform.stdio-convert-newlines` was still false - `mbed_error`
crashes would be garbled.

This wasn't usually spotted because applications generally have both
settings false or both true, and if newline conversion is on, then
`mbed_error_puts` writes 1 character at a time to FileHandle::write,
avoiding the length error.

drivers/UARTSerial.cpp

@@ -147,7 +147,6 @@ ssize_t UARTSerial::write_unbuffered(const char *buf_ptr, size_t length)
 
     for (size_t data_written = 0; data_written < length; data_written++) {
         SerialBase::_base_putc(*buf_ptr++);
-        data_written++;
     }
 
     return length;