stmmac: enable/disable rx/tx in the core with a single write.

This patch enables and disables the rx and tx bits in the MAC control reg
by using a single write operation.
This also solves a possible problem (spotted on SPEAr platforms) at 10Mbps
where two consecutive writes to a MAC control register can take more than
4 phy_clk cycles.

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