net: stmmac: Enable EEE HW LPI timer with auto SW/HW switching

net: stmmac: Enable EEE HW LPI timer with auto SW/HW switching

This patch enables the HW LPI Timer which controls the automatic entry
and exit of the LPI state.
The EEE LPI timer value is configured through ethtool. The driver will
auto select the LPI HW timer if the value in the HW timer supported range.
Else, the driver will fallback to SW timer.

Signed-off-by: Vineetha G. Jaya Kumaran <vineetha.g.jaya.kumaran@intel.com>
Signed-off-by: Voon Weifeng <weifeng.voon@intel.com>
Link: https://lore.kernel.org/r/20201027160051.22898-1-weifeng.voon@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>