net: dsa: xrs700x: fix missing initialisation of ds->phylink_mac_ops

The kernel build bot identified the following mistake in the recently
merged 860a9bed

 ("net: dsa: xrs700x: provide own phylink MAC
operations") patch:

drivers/net/dsa/xrs700x/xrs700x.c:714:37: warning: 'xrs700x_phylink_mac_ops' defined but not used [-Wunused-const-variable=]
     714 | static const struct phylink_mac_ops xrs700x_phylink_mac_ops = {
         |                                     ^~~~~~~~~~~~~~~~~~~~~~~

Fix the omitted assignment of ds->phylink_mac_ops.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>