iwlagn: fix debug variable access

The compiler correctly warns:
iwl-agn-tx.c: In function ‘iwlagn_tx_status_reply_compressed_ba’:
iwl-agn-tx.c:1240: warning: ‘bitmap’ may be used uninitialized in this function

Move the debug print to the branch that reads the
bitmap, and move the variables too so it's more
obvious where they are needed.

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