stmmac: rework synopsys id read, moved to dwmac setup

synopsys_uid is only used once after setup, to get synopsys_id
by using shitf/mask operation. It's no longer used then.
So, remove this temporary variable and directly compute
synopsys_id from setup routine.

Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>