wifi: iwlwifi: mvm: Fix getting the lowest rate

When setting the rate for Tx, the code tried to get the lowest
allowed rate but without considering the BSS basic rates. Fix this
by considering the basic rates.

In addition, declare support for configuring beacon Tx rate and when
configured use the configured Tx beacon rate to set beacon tx command rate.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20221102165239.be8403f0d5c0.I7d141646746b96310efd75fc77ca9aebc61aefcc@changeid