wifi: rtlwifi: cleanup USB interface

Drop unused '_usb_writen_sync()' and relevant pointer
from 'struct rtl_io', handle possible write error in
'_usb_write_async()', adjust related code.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230614061832.40882-2-dmantipov@yandex.ru