iwlwifi: fix bugs in beacon configuration

When sending beacon commands to the uCode, we must
inform it of the offset in the beacon frame of the
TIM Element so it can transmit packets from the
correct queue. This functionality is implemented
in iwl_set_beacon_tim().

Fix a bug setting the rate_n_flags for the beacon
packet. First, it should not use the station table's
rate (it's a management frame), and second it needs
to properly configure the TX antennas.

Finally, also, clean up and comment relevant functions.

Signed-off-by: Daniel C Halperin <daniel.c.halperin@intel.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>