net/designware: Consecutive writes must have delay

This patch solves a TX/RX problem which happens at 10Mbps, due to the
fact that we are not respecting 4 cyles of the phy_clk (2.5MHz) between
two consecutive writes on the same register.

Signed-off-by: Armando Visconti <armando.visconti@st.com>
Signed-off-by: Amit Virdi <amit.virdi@st.com>