iwlagn: sparse warning priv->temperature is signed

Since priv->temperature is signed, we cannot use debugfs_create_u32
to refer to it.
Use a regular debugfs file instead.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>