stmmac: allow mtu bigger than 1500 in case of normal desc (V4)

This patch allows to set the mtu bigger than 1500
in case of normal descriptors.
This is helping some SPEAr customers.

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