rt2x00: Always enable TSF ticking

Whatever mode we are in, according to the legacy drivers
we should always enable TSF ticking/counting.
We should also always enable the TBCN/TBTT field,
this field is only disabled during beacon regeneration.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>