iwlagn: add feature flags

Some new devices and microcode files will a greater
variety of features, so the TLV-per-feature approach
we took before will quickly make things harder to
manage and increase the file size.

Add a new TLV that has feature flags. Currently, it
will contain:
 1) a PAN feature flag, which moves from a separate
    TLV
 2) a new BT stats bit that indicates whether the
    microcode image uses bluetooth statistics
 3) a new MFP flag for management frame protection
    which can be enabled once the device/microcode
    supports it

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>