Experimental support for fragmentation and RTS/CTS.

Experimental support for fragmentation and RTS/CTS.

Delete unused atw_voodoo and constants.

Export Tx/Rx statistics with evcnt(9).

Correct the Short Inter-Frame Space (SIFS) that we write to ADM8211's
registers; I do not recall if that corrected the SIFS that I observed
"on the air."  Use the constant IEEE80211_DUR_DS_EIFS to configure
the ADM8211's EIFS, instead of writing the same "magic" number,
0x64, that my reference driver wrote.

Do not clear OACTIVE in atw_init(), because atw_stop() cleared it
previously by calling atw_txdrain().

Use the net80211 short-preamble flag and instead of ATW_SHPREAMBLE.

Add an ADM8211 workaround from the reference driver, atw_workaround1(),
but don't compile it right now.

In at_intr(), don't stop processing the interrupt status after
restarting the receive ring, but process Tx interrupt status.  If
a packet's Tx lifetime is exceeded, reinitialize the device to get
packets moving again.  If the Tx FIFO underflows, restart the
transmitter, not the receiver!

Avoid losing synchronization with the Rx ring by replicating one
of Charles Hannum's fixes to rtw(4) here: receiving a management
packet may, as a side-effect, reset the Rx ring, so refer to the
softc's Rx ring pointer, sc_rxptr, every time through the loop in
atw_rxintr(), instead of refering to a pointer on the stack, i.

Re-synchronize DMA after reading the OWN bit on an Rx/Tx descriptor.
XXX This needs more work.

Reset sc_tx_timer as Tx descriptors are reclaimed from the device.

Shorten staircases in atw_watchdog().

Remove from softc an unused member, sc_intr_ack.