net: phy: micrel: set soft_reset callback to genphy_soft_reset for KSZ8081

Following a similar reinstate for the KSZ9031.

Older kernels would use the genphy_soft_reset if the PHY did not implement
a .soft_reset.

Bluntly removing that default may expose a lot of situations where various
PHYs/board implementations won't recover on various changes.
Like with this implementation during a 4.9.x to 5.4.x LTS transition.
I think it's a good thing to remove unwanted soft resets but wonder if it
did open a can of worms?

Atleast this fixes one iMX6 FEC/RMII/8081 combo.

Fixes: 6e2d85ec

 ("net: phy: Stop with excessive soft reset")
Signed-off-by: Christian Melki <christian.melki@t2data.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20210224205536.9349-1-christian.melki@t2data.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>