Pre-11n devices and AR5416 use AR_PHY(263) for current RX RSSI.
AR9130 and later have a fourth calibration register (for doing
ADC calibration) and thus the register has moved to AR_PHY(271).
This isn't currently used by any of the active code; I'm committing
this for completeness and in case any third party code attempts to
use it for legacy reasons.