ibm_newemac: Fix dangerous struct assumption

The ibm_newemac driver includes code which assumes that the
work_struct which is included in every delayed_work is the first
member of that structure. This is currently the case but might change
in the future, so use to_delayed_work() instead, which doesn't make
such an assumption.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>