stmmac: release tx lock, in case of dma mapping error.

Add missing spin_unlock when tx frames gets dropped.

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