ath6kl: create ath6kl_core.ko

Now ath6kl is ready for splitting core code to ath6kl_core.ko module.
This also makes it possible to link both sdio and usb code to kernel
at the same time, which earlier failed miserably.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>