sfc: Clean up MDIO flag setting

We often want to set or clear a flag in an MDIO register, but avoid
writing if no change is required since this can have side-effects.
Encapsulate this in a function, mdio_clause45_set_flag().

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>