iwmc3200wifi: cache keys when interface is down

When the interface is down and one sets a WEP key from userspace, we should
be able to simply cache it.
Since that implies setting part of the profile's security settings, we now
alloc/free the umac_profile at probe/remove time, and no longer at interface
bring up/down time. Simply resetting it during the latter is enough.

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