ath6kl: convert a driver to use module_usb_driver()

This converts a driver in drivers/net/* to use the
module_usb_driver() macro which makes the code smaller and a
bit simpler.

Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>