iwlwifi: fix possible priv->mutex deadlock during suspend

This patch moves _cancel_deferred_work out of mutex protection and removes
unnecessary mutex in pci_suspend and pci_resume.

Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>