iwmc3200wifi: make iwm_send_wifi_if_cmd return 0 on success

We used to return the result of wait_event_interruptible_timeout()
which is the remaining timeout on success. But this information is
not used by any of its callers. So we just return 0 on success.
This fixed a erroneous return value bug for iwm_set_key().

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