net: stmmac: Modify configuration method of EEE timers

Ethtool manual stated that the tx-timer is the "the amount of time the
device should stay in idle mode prior to asserting its Tx LPI". The
previous implementation for "ethtool --set-eee tx-timer" sets the LPI TW
timer duration which is not correct. Hence, this patch fixes the
"ethtool --set-eee tx-timer" to configure the EEE LPI timer.

The LPI TW Timer will be using the defined default value instead of
"ethtool --set-eee tx-timer" which follows the EEE LS timer implementation.

Changelog V2
*Not removing/modifying the eee_timer.
*EEE LPI timer can be configured through ethtool and also the eee_timer
module param.
*EEE TW Timer will be configured with default value only, not able to be
configured through ethtool or module param. This follows the implementation
of the EEE LS Timer.

Fixes: d765955d ("stmmac: add the Energy Efficient Ethernet support")
Signed-off-by: Vineetha G. Jaya Kumaran <vineetha.g.jaya.kumaran@intel.com>
Signed-off-by: Voon Weifeng <weifeng.voon@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>