r8188eu: remove unused urbs from struct xmit_buf

Driver allocates eighth urbs per xmit_buf, but uses only first urb.
Allocation of seven remaining urbs wastes memory for nothing.

Reviewed-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Ivan Safonov <insafonov@gmail.com>
Link: https://lore.kernel.org/r/20220417163608.42917-1-insafonov@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>