net: stmmac: split the stmmac_adjust_link 10/100 case

The 10/100 case have too many ifcase.
This patch split it for removing an if.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Reviewed-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>