ath10k-firmware: update firmware version for QCA988X (again)

Updates the ath10-firmware release to current git and updates the
QCA988X firmware in use to version 10.2.4.70.12-2 which appears to
actually fix the stability problems when using a Nexus 5X phone as
per issue #20854. Also updates the Makefile for a change in the
location of the board.bin file for QCA988X in the source repository.

Signed-off-by: Robert Hancock <hancockrwd@gmail.com>

SVN-Revision: 47469