avr_register_io_[read,write](): fail if overriding earlier registration.

avr_register_io_[read,write](): fail if overriding earlier registration.

The avr_register_io_*() functions do not maintain a chain of
callbacks. This can be confusing if someone like me tries to use them
to listen to some I/O events (for which you should use the avr_irq_*
stuff).

Make this more explicit by checking in the register functions if we
already have a callback and failing with an error message if so.

Signed-off-by: Sami Liedes <sliedes@cc.hut.fi>