ath9k_hw: fix chain swap setting when setting rx chainmask to 5

Chain swapping should only be enabled when the EEPROM chainmask is set to 5,
regardless of what the runtime chainmask is.

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