ath5k: MRR support and 2GHz radio override belong in ah_capabilities

MRR support and 2GHz radio override belong in ah_capabilities and we
should use them (e.g. so far  we used to set mrr descriptor without
checking if MRR support is enabled + we checked for MRR support 2
times, one by trying to set up an MRR descriptor and another one based
on MAC version).

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>