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

commit 24171dd92096fc370b195f3f6bdc0798855dc3f9 upstream.

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

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
[bwh: Backported to 3.2: keep the special case for AR_SREV_9462 here]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>