net: atlantic: update flow control logic

We now differentiate requested and negotiated flow control
modes. Therefore `ethtool -A` now operates on local requested
FC values, and regular link settings shows the negotiated FC
settings.

Signed-off-by: Nikita Danilov <ndanilov@marvell.com>
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>