nl80211: add testmode dump support

This adds dump support to testmode. The testmode
dump support in nl80211 requires using two of the
six cb->args, the rest can be used by the driver
to figure out where the dump position is at or to
store other data across invocations.

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