altos: Need to use 16-bit counts for ao_xmem functions

Trying to use 8-bit counts is a nice optimization which fails when the
count is larger than 255, as is the case with clearing the flash block
in the AT45 driver. This bug resulted in the inability to erase
flights on TeleMetrum v1.0 boards.

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