ath6kl: add back beginnings of USB support

John Linville had to revert the part of USB support which was already
in ath6kl due to build problems in commit cb00ec38 ("ath6kl: revert
USB support"). Now that I fixed the build problems properly by adding
ath6kl_core.ko kernel module it's possible to add back the
(incomplete) USB support. This patch is a revert of John's patch and
adds back the USB code which as already in ath6kl, only difference
being minor changes in Makefile and adapting usb.c to new core
function names.

Note that USB support in ath6kl is not complete yet. This code only
makes it possible to boot firmware but as HTC layer does not yet
support USB it's not possible to send any WMI commands nor data
packets to the firmware. That will be added soon.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>