ath9k: avoid accessing MRC registers on single-chain devices

commit a1c781bb20ac1e03280e420abd47a99eb8bbdd3b upstream.

They are not implemented, and accessing them might trigger errors

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