net: phy: micrel: add Asym Pause workaround

The Micrel KSZ9031 PHY may fail to establish a link when the Asymmetric
Pause capability is set. This issue is described in a Silicon Errata
(DS80000691D or DS80000692D), which advises to always disable the
capability. This patch implements the workaround by defining a KSZ9031
specific get_feature callback to force the Asymmetric Pause capability
bit to be cleared.

This fixes issues where the link would not come up at boot time, or when
the Asym Pause bit was set later on.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>