iwmc3200wifi: add some more range checks

My previous patch added a check to get_key() but missed a couple
other places which need range checks.

The problem here is that wifi drivers have different numbers of keys.
The lower levels assume that they can have up to 4 default keys and
2 management keys but this driver only has the default keys so we
could go past the end of the ->keys[] array.

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>