iwlagn: support new P2P implementation

The previous P2P implementation turned out to
not work well and new uCode capabilities were
added to support P2P. Modify the driver to
take advantage of those, and also discover P2P
support automatically based on a uCode flag
instead of having a Kconfig symbol for P2P.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>