iwlwifi: allocated rx page accounting cleanup

In iwlwifi, priv->alloc_rxb_page is used to keep track of the Rx
pages allocated by the driver. This cleans up the page free routines
by introducing __iwl_free_pages/iwl_free_pages so that the accounting
is more accurate and less error prone. This also fixes two instances where
the counter was not updated.

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>