Move slot-time, CTS/RTS, and ACK timeout calculations into the driver. Corrected...

Move slot-time, CTS/RTS, and ACK timeout calculations into the driver.  Corrected logic for the relationships between slot time and cts timeout and ack timeout have been added and the athctrl now delegates to the driver instead.

You can override:
- slottime
- ctstimeout
- acktimeout

acktimeout and ctstimeout will be selected from the first available of:

1) the explicitly specified override value [if present]
2) a value derived from an explicitly specified slottime [if present]
3) the HALs default behavior / standard settings for the PHY mode

Setting the distance is shorthand for updating the slottime, and both cts and ack timeout values based upon the usual equations for air propagation, speed of light, etc..etc..

git-svn-id: http://madwifi-project.org/svn/madwifi/trunk@3508 0192ed92-7a03-0410-a25b-9323aeb14dbd