IB/ipath: Fix crash on unload introduced by sysfs changes

Commit 23b9c1ab ("Infiniband: make ipath driver use default driver
groups.") introduced a bug in the ipath driver where
ipath_device_create_group() fell through into the error path, even on
success, which meant that the sysfs groups it created would always get
removed right away.  This made ipath_device_remove_group() hit the
BUG_ON() in sysfs_remove_group() when it tried to remove those groups a
second time.

Correct the return path so that the groups stick around until they are
supposed to be cleaned up.

Signed-off-by: Roland Dreier <rolandd@cisco.com>