stmmac: fix max coal timer parameter

This patch is to fix the max coalesce timer setting that can be provided
by ethtool.
The default value (STMMAC_COAL_TX_TIMER) was used in the set_coalesce helper
instead of the max one (STMMAC_MAX_COAL_TX_TICK, so defined but not used).

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