wireless: move some utility functions from mac80211 to cfg80211

The patch moves some utility functions from mac80211 to cfg80211.
Because these functions are doing generic 802.11 operations so they
are not mac80211 specific. The moving allows some fullmac drivers
to be also benefit from these utility functions.

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