net: stmmac: add stmmac_try_to_start_sw_lpi()

There are two places which call stmmac_enable_eee_mode() and follow it
immediately by modifying the expiry of priv->eee_ctrl_timer. Both code
paths are trying to enable LPI mode. Remove this duplication by
providing a function for this.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1tXItM-000MBI-CX@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>