mac82011: Allow selection of minstrel_ht as default rc algorithm

Allow selection of minstrel_ht as default rate control algorithm. At
the moment minstrel_ht can only be requested by the driver code but
not selected as default in make menuconfig. Fix this by using
minstrel_ht when minstrel was selected as default and minstrel_ht
is available.

This change won't affect legacy devices as minstrel_ht falls back to
minstrel in that case.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>