ath6kl: Return error case when ath6kl_usb_alloc_pipe_resources fails

Incase the resource allocation for the struct ath6kl_urb_context in the
function ath6kl_usb_alloc_pipe_resources fails, return this error
case so that ath6kl_usb_probe is aware of this error case.

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>