net: lwip: call sys_check_timeouts and schedule on rx

net: lwip: call sys_check_timeouts and schedule on rx

Call schedule() in net_lwip_rx() to service U-Boot tasks and
actions during packet rx.

As a cleanup also move sys_check_timeouts() here and remove it from the
functions that call net_lwip_rx().

This resolves the issue of an active watchdog resetting the board on
long network activities.

Suggested-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>