Fix read access for 16bit timer counter registers.

[ydirson]

To get meaningful reading, we must read the low byte
first, so the MCU saves the correct high byte value in
the counter's temporary register (see eg. section 14.2
of the 32u4 datasheet).

[ydirson]

Tested on my 32u4 board, on a project where I emulate several clocks on a single timer, and thus have to save/restore the timer state. Current getCounter() method just does not work.

I hope all AVR8 mcu's share the same behaviour.