iwlagn: remove unnecessary type for tracing operations

The device tracing routines only use the priv pointer as an opaque
value.  Change from a typed iwl_priv pointer to a null pointer and
eliminate the need to include iwl_priv.h.  CMD_ASYNC is defined in
iwl_shared.h which is the only reason it is included.

Signed-off-by: Don Fry <donald.h.fry@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>