ath10k: add initial USB support

Chipsets like QCA9377 have support for USB so add initial USB bus
support to ath10k. With this patch we have the low level HIF and
HTC protocol working and it's possible to boot the firmware,
but it's still not possible to connect or anything like.

More changes are needed for full functionality. For that reason
we print during initialisation:

WARNING: ath10k USB support is incomplete, don't expect anything to work!

Signed-off-by: Erik Stromdahl <erik.stromdahl@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>