altos: Use all 16 bits of setup packet len when limiting reply len

We were only using the low 8 bits of the setup packet reply max len,
which meant that if the other side sent a weird max len (as Windows 7
does), then we'd truncate our setup reply to whatever was in the low 8
bits of that value.

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