rt2x00: Use correct TBTT_SYNC config in AP mode

This seems to fix problems with some powersaving clients since a
positive value in TBTT_SYNC_CFG_TBTT_ADJUST introduces beacon skew,
which is not wanted in AP mode.

Also update the rest of the TBTT_SYNC config according to the
legacy drivers in AP mode.

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