atl1c: remove PHY polling from atl1c_change_mtu

PHY polling code for FPGA is considered in every MDIO R/W API.
no need to add additional code to atl1c_change_mtu.

Signed-off-by: xiong <xiong@qca.qualcomm.com>
Tested-by: David Liu <dwliu@qca.qaulcomm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>