net/macb: Only adjust tx_clk on link change

The PHY state machine (in drivers/net/phy/phy.c) will unconditionally
call phydev->adjust_link (macb_handle_link_change) when polling in the
PHY_CHANGELINK state. As currently written, macb always ends up
requesting a new tx_clk frequency in macb_handle_link_change. It is a
waste of time to request a new tx_clk frequency if the link state hasn't
changed, as the tx_clk will already be configured properly.

Let's only request a new tx_clk clock frequency when necessary.

Signed-off-by: Jaeden Amero <jaeden.amero@ni.com>
Cc: Josh Cartwright <joshc@ni.com>
Cc: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>