net: bcmgenet: add support for ethtool flow control

This commit extends the supported ethtool operations to allow MAC
level flow control to be configured for the bcmgenet driver.

The ethtool utility can be used to change the configuration of
auto-negotiated symmetric and asymmetric modes as well as manually
configuring support for RX and TX Pause frames individually.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>