ath6kl: create core.c

ath6kl: create core.c

Currently core functions are spread between various files, group all
the functions into file and rename the functions to follow the style
used elsewhere in the driver. This will make it easier to a separate core
module.

Also fix a bug where wiphy is freed too early.

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