iwlwifi: move NIC init and Tx queues init to iwlcore

This patch does the following:

1 - change hw_nic_init from a handler to a function
2 - move hw_nic_init function to iwlcore
3 - open a new file - iwl-tx.c
4 - move all Tx queues initialization (part of NIC init) to iwl-tx.c
5 - move iwl_rx_init, previously as part of the NIC init, to iwl-rx.c
6 - iwl4965_tfd_frame rename to iwl_tfd_frame

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