iwlwifi: fix locking assertions

spin_is_locked() can return zero on some (UP?)
configurations because locks don't exist, and
that causes an endless amount of warnings. Use
lockdep_assert_held() instead, which has two
advantages:
 1) it verifies the current task is holding
    the lock or mutex
 2) it compiles away completely when lockdep
    is not enabled

Cc: stable@kernel.org [2.6.34+, maybe only parts of patch]
Reported-by: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>