lan78xx: Avoid spurious kevent 4 "error"

lan78xx_defer_event generates an error message whenever the work item
is already scheduled. lan78xx_open defers three events -
EVENT_STAT_UPDATE, EVENT_DEV_OPEN and EVENT_LINK_RESET. Being aware
of the likelihood (or certainty) of an error message, the DEV_OPEN
event is added to the set of pending events directly, relying on
the subsequent deferral of the EVENT_LINK_RESET call to schedule the
work.  Take the same precaution with EVENT_STAT_UPDATE to avoid a
totally unnecessary error message.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>