usb_calc_bus_time — approximate periodic transaction time in nanoseconds
from dev->speed; USB_SPEED_{LOW,FULL,HIGH}
true iff the transaction sends data to the host
true for isochronous transactions, false for interrupt ones
how many bytes in the transaction.
Returns approximate bus time in nanoseconds for a periodic transaction. See USB 2.0 spec section 5.11.3; only periodic transfers need to be scheduled in software, this function is only used for such scheduling.