This adds a fastpath for the common workloads to the
MAC suspend flushing.
In common workloads the FIFO flush will take between 100 and
200 microseconds. So we want to avoid calling msleep() in the
common case, as it will waste over 800 microseconds + scheduler
overhead.

This fastpath will hit in workloads where only small chunks
of data are transmitted (downloading a file) or when a TX rate bigger
or equal to 24MBit/s is used when transmitting lots of stuff (iperf).
So in the commonly used workloads it will basically always hit.

In case the fastpath is not hit, there's no real performance or latency
disadvantage from that.

And yes, I measured this. So this is not one of these
bad Programmer Likeliness Assumptions that are always wrong. ;)

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>