If the I/O ports are non default (3f8), we had to specify manually the correct
I/O ports. With this commit the BDA is read and the I/O port of the first
serial interface (COM) is taken. If no serial interface is available no device
configuration will be undertaken.
The line-status register has two relevant status bits - transmitter-hold
register empty and data-hold register empty - from which only the THR is
relevant as it signals new character can be written to the device.
Fixes#281
