ath5k: write beacon control register twice when resetting tsf

According to the newly-released Atheros HAL code, asserting the
TSF reset bit will toggle a hardware internal state, resulting in a
spurious reset on the next chip reset.  Whenever we force a TSF bit,
write the bit twice to clear the internal signal.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>