net: fec: Add a fec_enet_clear_ethtool_stats() stub for CONFIG_M5272

Commit 2b30842b ("net: fec: Clear and enable MIB counters on imx51")
introduced fec_enet_clear_ethtool_stats(), but missed to add a stub
for the CONFIG_M5272=y case, causing build failure for the
m5272c3_defconfig.

Add the missing empty stub to fix the build failure.

Fixes: Commit 2b30842b

 ("net: fec: Clear and enable MIB counters on imx51")
Reported-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>