Add ath_hal_setbeacontimers() to the AR9300 HAL.

Add ath_hal_setbeacontimers() to the AR9300 HAL.

This is a custom FreeBSD HAL method that is used by the TDMA code
to program the beacon timers directly without any guesswork/assumptions
by the HAL.

This brings up basic TDMA master/slave support on the AR9380 HAL,
however there are other issues preventing it from being stable.
(I'm seeing beacon interval instability, which may be due to
busy 2GHz air, but also may be due to some HAL configuration
issues with regards to ANI, or hardware timer programming, etc.)

Tested:

* AR9331 (Carambola2), STA, AP, adhoc and TDMA master mode.