Fix osccalASM.S to work when connected to hub

A hub can broadcast USB packets intended for other devices to micronucleus just after the USB reset pulse, before osccalASM.S has had time to calibrate OSCCAL based on the 1kHz keepalive pulses. On one of my machines osccalASM.S was getting confused by these packets and thinking they were really closely spaced keepalive pulses, so coming up with a wildly wrong OSCCAL value and causing V-USB not to work. This change detects USB packets during this time and avoids using that 1mS interval for calibration.

cpldcpu has commented in email that this fix might not be sufficient as heavy hub activity just after reset could cause this new osccalASM.S to keep ignoring every USB frame (due to data packets) and never finish, with the host eventually starting to try to communicate with micronucleus and timing out.