[PATCH] ath5k: Internalize Atheros Turbo modes

This patch:

* Internalizes Atheros Turbo modes (MODE_ATHEROS_TURBO,
MODE_ATHEROS_TURBOG)

* Internatizes Turbo modulation (MODULATION_TURBO)

* Adds documention for Atheros Turbo modes (MODE_ATHEROS_TURBO,
MODE_ATHEROS_TURBOG), Turbo modulation (MODULATION_TURBO) and XR
modulation (MODULATION_XR)

Changes-licensed-under: BSD
Signed-off-by: Luis R. Rodriguez <mcgrof@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>