stmmac: don't go through ethtool to start auto-negotiation

The driver used to call phy's ethtool configuration routine to start
auto-negotiation. This change has it call directly phy's routine to
start auto-negotiation.

The initial version was hiding phy_start_aneg() return value,
this patch returns it (<0 upon error).

Tested: module compiles, tested on STM HDK7108 STB.

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