ath9k: Use minstrel rate control by default

The ath9k rate control algorithm has various architectural
issues that make it a poor fit in scenarios like congested
environments etc.

An example: https://bugzilla.redhat.com/show_bug.cgi?id=927191

Change the default to minstrel which is more robust in such cases.
The ath9k RC code is left in the driver for now, maybe it can
be removed altogether later on.

Cc: stable@vger.kernel.org
Cc: Jouni Malinen <jouni@qca.qualcomm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>