* add support to read the timer and capability
* add support to enable/disable the location timer.
On AR9380 at least, enabling the location timer is required to make
the timer tick, otherwise location packets return a timestamp of 0.
However, it then makes /all/ RX packets use the RX location timestamp
instead of the TSF timestamp.
So, unless I find another magical way to do location timestamping,
we will have to dynamically switch things on/off and ensure the
TX/RX path handles the "different" timestamps correctly.
Tested:
* AR9380, STA mode
e6df5ff544