A regression introduced in https://github.com/raspberrypi/linux/pull/3887
meant that if the newly scheduled transfer immediately returned data, and
the driver resubmitted a single URB after every transfer, then the effective
polling interval would end up being approx 1ms.

Use the larger of SCHEDULE_SLOP or the configured endpoint interval.

Signed-off-by: Jonathan Bell <[email protected]>