ath6kl: check usb_register() return value

ath6kl_usb_init() does not check usb_register() return value.
As a result it may incorrectly report success of driver initialization.

Found by Linux Driver Verification project (linuxtesting.org).

kvalo: fix commit title and make cosmetic changes to the code to follow
more the style used in the driver

Signed-off-by: Marina Makienko <makienko@ispras.ru>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>