net: stmmac: clean up stmmac_disable_eee_mode()

net: stmmac: clean up stmmac_disable_eee_mode()

stmmac_disable_eee_mode() is now only called from stmmac_xmit() when
both priv->tx_path_in_lpi_mode and priv->eee_sw_timer_en are true.
Therefore:

	if (!priv->eee_sw_timer_en)

in stmmac_disable_eee_mode() will never be true, so this is dead code.
Remove it, and rename the function to indicate that it now only deals
with software based EEE mode.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Choong Yong Liang <yong.liang.choong@linux.intel.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1tVZDw-0002KL-Gg@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>