ath10k-firmware: update the qca988x firmware to 10.2.4.70.54

Use firmware version 10.2.4.70.54 from kvalo's git repository. The old
version (even though it's version number is greater) is an old version
from September 2015.
Using only the firmware versions from kvalo's git repo is recommended,
because those are tested by QCA's internal QCA.

The QCA988X directory received a small reorganization as a "hw2.0"
subdirectory was added - this patch also takes care of that as
board.bin was moved to that subdirectory.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>