net: bcmgenet: only update UMAC_CMD if something changed

The link adjustment callback can be called as frequently as desired
by the PHY library, as such, let's avoid doing a Read/Modify/Write
sequence if nothing changed, since these register accesses can be
expensive.

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