cuda.c: return error for unknown commands

This avoids MacsBug hanging at startup in the absence of ADB mouse
input, by replying with an error (which is also what MOL does) when
it sends an unknown command (0x1c).

Signed-off-by: Alyssa Milburn <fuzzie@fuzzie.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
