firmware: Do not use WARN_ON(!spin_is_locked())

spin_is_locked() always returns false for uniprocessor configurations
in several architectures, so do not use WARN_ON with it.
Use lockdep_assert_held() instead to also reduce overhead in
non-debug kernels.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: <stable@vger.kernel.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>