ath9k: do not limit the chainmask to 1 for legacy mode

Restricting the chainmask to 1 for legacy mode disables useful features
such as MRC, and it reduces the available transmit power.
I can't think of a good reason to do this in legacy mode, so let's just
get rid of that code.

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