p54: honour bss_info_changed's basic_rates and other settings

As was pointed out in "p54: honour bss_info_changed's short slot time settings",
bss_info_changed provides more useful settings that can be used by the driver.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>