stmmac: fix stmmac_tx_avail should be called with TX locked

stmmac_tx_avail() may lie if used unprotected. It's using cur_tx
and dirty_tx index. These index may be already in use by tx_clean
when entering xmit routine. So, this should be called locked.

This can cause transmit queue to be stuck, with following message:
NETDEV WATCHDOG: eth0 (stmmaceth): transmit queue 0 timed out

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>