net: stmmac: fix LPI transitioning for dwmac4

The LPI transitioning logic in stmmac_main uses
priv->tx_path_in_lpi_mode to enter/exit LPI.

However, priv->tx_path_in_lpi_mode is assigned
using the return value from host_irq_status().

So for dwmac4, priv->tx_path_in_lpi_mode was always false,
so stmmac_tx_clean() would always try to put us in eee mode,
and stmmac_xmit() would never take us out of eee mode.

To fix this, make host_irq_status() read and return the LPI
irq status also for dwmac4.

This also increments the existing LPI counters, so that
ethtool --statistics shows LPI transitions also for dwmac4.

For dwmac1000, irqs are enabled/disabled using the register
named "Interrupt Mask Register", and thus setting a bit disables
that specific irq.

For dwmac4 the matching register is named "MAC_Interrupt_Enable",
and thus setting a bit enables that specific irq.

Looking at dwmac1000_core.c, the irqs that are always enabled are:
LPI and PMT.

Looking at dwmac4_core.c, the irqs that are always enabled are:
PMT.

To be able to read the LPI irq status, we need to enable the LPI
irq also for dwmac4.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>