altos: ao_packet_getchar needs to return 'int' so AO_READ_AGAIN works

With ao_packet_getchar returning 'char', AO_READ_AGAIN would get
converted to 0xff instead of -1 on return, making ao_packet_echo
either produce a bogus character on output, breaking command
responses, or wedge and never exit when the link was dropped.

Signed-off-by: Keith Packard <keithp@keithp.com>