net: dsa: ar9331: Use explict flags for regmap single read/write

The at9331 is only able to read or write a single register at once.  The
driver has a custom regmap bus and chooses to tell the regmap core about
this by reporting the maximum transfer sizes rather than the explicit
flags that exist at the regmap level.  Since there are a number of
problems with the raw transfer limits and the regmap level flags are
better integrated anyway convert the driver to use the flags.

No functional change.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>