iwlwifi: add remove station functionality

This patch adds remove station functionality, which is required for
5000 and AP mode.

There are still some gaps in managment that need to be closed but it
provides sufficient functionality for 5000 HW.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>