ath9k_hw: start documenting 802.11n RF anlong front ends

Document what we can about the RF analog front ends (radios)
of Atheros 802.11n devices. What should be clearer now is the
what we do for old pre AR5416 and AR5418 MAC based devices in
comparison to the modern sigle-chip 802.11n solutions.

All devices after AR9280 are single chip and require less
programming -- the RF registers no longer need to be initialized
as they all have the RF analog front end embedded together with
the MAC/BB; this includes the AR9271. Older devices such as the
ones with the AR5416 MACs (PCI) or AR5418 MACs (PCI-E) have an
external 2.4 GHz AR2133 radio or a dual band 2.4 GHz / 5 GHz
AR5133 radio. These external radios require additional programming
of the RF registers.

Clarify which parts are for what devices and which code is
shared. This patch has no functional changes.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>