net/phy: extra delay only for RGMII interfaces for IC+ IP 1001

The extra delay of 2ns to adjust RX clock phase is actually needed
in RGMII mode. Tested on the HDK7108 (STx7108c2).

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>