altos: reorder stm USB state stores to avoid races

altos: reorder stm USB state stores to avoid races

Must set ao_usb_in_pending before telling USB about new data or an
interrupt could arrive at the wrong time to clear it.

Same for ao_usb_in_flushed.

Without these changes, I've seen the USB bus lock up on occasion,
waiting for an IN packet to consume data, but with no IN data pending
in the hardware.

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