ath9k_hw: fix calibration on 5 ghz

ADC calibrations cannot run on 5 GHz with fast clock enabled. They
need to be disabled, otherwise they'll hang and IQ mismatch calibration
will not be run either.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Reported-by: Adrian Chadd <adrian@freebsd.org>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>