iwlwifi: add bad length check for WEP keys

This patch adds a check for bad length in set key flow. This solves the
Oops reported by Thomas Backlund, Joonwoo Park and Ian Schram.
It also adds some debug printing that can be useful.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>