iwlwifi: remove 4965 from iwl4965_tx_queue_update_write_ptr

This patch renames iwl4965_tx_queue_update_write_ptr to
iwl_txq_update_write_ptr. This is a preparation for moving to iwl-tx.c.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>