ath9k_hw: validate and fix broken eeprom chainmask settings

Some devices (e.g. Ubiquiti AirRouter) ship with broken EEPROM chainmask
data, which breaks the initial calibration after a hardware reset.
To fix this, mask the eeprom chainmask with the chainmask of the chip,
and use the chip chainmask if the result is zero.

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