Merge branch 'dsa-mv88e6xxx-Improve-indirect-addressing-performance'

Tobias Waldekranz says:

====================
net: dsa: mv88e6xxx: Improve indirect addressing performance

The individual patches have all the details. This work was triggered
by recent work on a platform that took 16s (sic) to load the mv88e6xxx
module.

The first patch gets rid of most of that time by replacing a very long
delay with a tighter poll loop to wait for the busy bit to clear.

The second patch shaves off some more time by avoiding redundant
busy-bit-checks, saving 1 out of 4 MDIO operations for every register
read/write in the optimal case.

v1 -> v2:
- Make sure that we always poll the busy bit at least twice, in the
  unlikely event that the first one is quick to query the hardware,
  but is then scheduled out for a long time before the timeout is
  checked.

v2 -> v3:
- Fallback to the longer sleeps after the initial two poll attempts.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>