iwlegacy: don't mess up the SCD when removing a key

When we remove a key, we put a key index which was supposed
to tell the fw that we are actually removing the key. But
instead the fw took that index as a valid index and messed
up the SRAM of the device.

This memory corruption on the device mangled the data of
the SCD. The impact on the user is that SCD queue 2 got
stuck after having removed keys.

Reported-by: Paul Bolle <pebolle@tiscali.nl>
Cc: stable@vger.kernel.org
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>