mac80211: make ath10k firmware v2 selectable

Added option for old firmware version (10.1 API v2). It seems that recent
firmware versions are constantly crashing (at least on a QCA9880-BR4A-R) and
this provides the option to select an older version instead.

Signed-off-by: Günther Kelleter <guenther.kelleter@devolo.de>

SVN-Revision: 46340