ath5k: do not re-run AGC calibration periodically

All other Atheros drivers run the AGC gain calibration and DC offset
calibration only after reset. Running them periodically has caused stability
issues on some (primarily AR2315/2413/5413/5414 based) devices, leading to
messages such as:

ath5k phy0: gain calibration timeout (2462MHz)
ath5k phy0: calibration of channel 11 failed

Related bug reports:
https://dev.openwrt.org/ticket/10574
https://bugzilla.redhat.com/show_bug.cgi?id=795141

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>