ath10k: Fix an error handling path

If 'ath10k_usb_create()' fails, we should release some resources and report
an error instead of silently continuing.

Fixes: 4db66499

 ("ath10k: add initial USB support")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201122170342.1346011-1-christophe.jaillet@wanadoo.fr