net: dsa: mv88e6xxx: Improve performance of busy bit polling

Avoid a long delay when a busy bit is still set and has to be polled
again.

Measurements on a system with 2 Opals (6097F) and one Agate (6352)
show that even with this much tighter loop, we have about a 50% chance
of the bit being cleared on the first poll, all other accesses see the
bit being cleared on the second poll.

On a standard MDIO bus running MDC at 2.5MHz, a single access with 32
bits of preamble plus 32 bits of data takes 64*(1/2.5MHz) = 25.6us.

This means that mv88e6xxx_smi_direct_wait took 26us + CPU overhead in
the fast scenario, but 26us + 1500us + 26us + CPU overhead in the slow
case - bringing the average close to 1ms.

With this change in place, the slow case is closer to 2*26us + CPU
overhead, with the average well below 100us - a 10x improvement.

This translates to real-world winnings. On a 3-chip 20-port system,
the modprobe time drops by 88%:

Before:

root@coronet:~# time modprobe mv88e6xxx
real    0m 15.99s
user    0m 0.00s
sys     0m 1.52s

After:

root@coronet:~# time modprobe mv88e6xxx
real    0m 2.21s
user    0m 0.00s
sys     0m 1.54s

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>