stmmac: fix LPI TW timer value to 20.5us.

The value for LPI TW timer has to be updated to 0x1E that is the hardcoded value
of 20.5us and it will apply to all EEE enabled Remote PHYs.
Disadvantage is for PHY's that support lesser wakeup time but we can accept it
waiting to implement LLDP to negotiate the Wakeup time of Remote PHY.

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