net: sun4i: fix timeout check

The current timeout check is comparing two constant values, so it won't
ever detect a timeout. This patch reworks the affected code a bit so it
has a chance at detecting timeouts correctly.

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>