ig4: Actively use FIFO thresholds

Before every wait for FIFO interrupt set how much data/space do we
want to see there. Previous code was not using it for receive, as
result aggregating interrupts only within processing latency. The
new code needs only one interrupt per transfer per FIFO length.

On my Dell XPS 13 9310 with iichid(4) touchscreen and touchpad this
reduces the interrupt rate per device down to 2 per sample or 16-20
per second when idle and 120-160 per second when actively touched.

MFC after: 1 month