IB/ehca: Fix device registration

Move the call to ib_register_device() later, since a device should not
be registered until it is completely read to be used.  This fixes
crashes that occur if an upper-layer driver such as IPoIB is loaded
before the ehca module.

Signed-off-by: Hoang-Nam Nguyen <hnguyen@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>