IPoIB: Always initialize poll_timer to avoid crash on unload

ipoib_ib_dev_stop() does del_timer_sync(&priv->poll_timer), but if a
P_key for an interface is not found, poll_timer is not initialized, so
this leads to a crash or hang.  Fix this by moving where poll_timer is
initialized to ipoib_ib_dev_init(), which is always called.

This fixes <https://bugs.openfabrics.org/show_bug.cgi?id=1172>.

Debugged-by: Yosef Etigin <yosefe@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>