pasemi_mac: Improve RX interrupt mitigation

Currently the receive side interrupts will go off on the reception of
a packet, NAPI will poll the ring and keep polling as long as there's
a decent amount of packets to receive.

This is less than optimal, especially for LRO where it's better if we
have a more substantial amount of packets to process at once, to get
the real LRO benefits.

So, set the count threshold to a higher value and use the timeout feature
that will give us an interrupt even if not enough packets have come in
to set off the count threshold.

FIXME: It'd be real nice to have ethtool support for users to tune this
at runtime.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>