stmmac: fix macro used for debugging the xmit

This patch fixes the name of the macro used for
debugging the transmit process. I used STMMAC_TX_DEBUG
instead of STMMAC_XMIT_DEBUG.

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