iwmc3200wifi: add iwm_if_add and iwm_if_remove

We used to do alloc_netdev and register_netdev at the same time in
iwm_if_alloc. But some bus related structures will only be initialized
after iwm_priv is allocated. This caused a race condition that the
netdev might be registered earlier. The patch adds iwm_if_add and
iwm_if_remove so that the bus layer could register the device after
all initialization is done.

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: Samuel Ortiz <samuel.ortiz@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>