net: stmmac: provide stmmac_eee_tx_busy()

Extract the code which checks whether there's still work to do on any
of the stmmac transmit queues. This will allow us to combine
stmmac_enable_eee_mode() with stmmac_try_to_start_sw_lpi() in the
next patch.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1tXItR-000MBO-GF@rmk-PC.armlinux.org.uk


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