iwmc3200wifi: add a range check to iwm_cfg80211_get_key()

Smatch complains that "key_index" is capped at 5 in nl80211_get_key()
but iwm->keys[] only has 4 elements.  I don't know if this is really
needed, but the other ->get_key() implementations seemed to check
for overflows so I've added a check here.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>