drivers/net/wireless/iwlwifi/iwl-tx.c: fix gcc-3.4.5 warning

drivers/net/wireless/iwlwifi/iwl-tx.c: In function `iwl_hw_txq_ctx_free':
drivers/net/wireless/iwlwifi/iwl-tx.c:410: warning: suggest explicit braces to avoid ambiguous `else'

Cc: Zhu Yi <yi.zhu@intel.com>
Cc: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>