net: stmmac: reduce unnecessary wakeups from eee sw timer

Currently, on EEE capable platforms, if EEE SW timer is used, the SW
timer cause 1 wakeup/s even if the TX has successfully entered EEE.
Remove this unnecessary wakeup by only calling mod_timer() if we
haven't successfully entered EEE.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>