drivers: auxdisplay: hd44780: implement busy-flag polling mechanism

HD44780 controller can indicate via busy flag whether it's finished
processing current command. This allows for faster completion of HD44780
commands as seen from MCU perspective, as the MCU doesn't have to wait
for fixed long period of time. Implement this functionality.

Signed-off-by: Jan Kuliga <jtkuliga@gmail.com>