realtek: Use atomic poll for aux-mdio commands

realtek: Use atomic poll for aux-mdio commands

regmap_read_poll_timeout() relies on usleep_range() to time the polling
loop. With the current, rather large, scheduling interval, a short
usleep_range() may take a lot longer than expected, causing performance
issues.

Switch the driver over to using regmap_read_poll_timeout_atomic(), which
uses udelay() to time the polling loop.

For comparision, the 'ethtool -m <dev>' command is about 10 times faster
with the atomic variant.
Using 'perf -r10 ethtool -m lan25':
  - Driver using regmap_read_poll_timeout():
      2.0117 +- 0.0118 seconds time elapsed  ( +-  0.58% )
  - Driver using regmap_read_poll_timeout_atomic():
      0.1674 +- 0.0250 seconds time elapsed  ( +- 14.95% )

Signed-off-by: Sander Vanheule <sander@svanheule.net>