rt2x00: Add missing TSF sync mode for AP operation

Currently rt2x00 uses the TSF_SYNC_BEACON mode for all beaconing
interface types. However, TSF_SYNC_BEACON is meant for IBSS networks and
thus implements TSF merging in the hardware. Rename TSF_SYNC_BEACON to
TSF_SYNC_ADHOC to better express its purpose and introduce the missing
TSF sync mode TSF_SYNC_AP_NONE which should be used for beaconing modes
that don't need TSF merging.

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>