USB: serial: spcp8x5: remove unused struct 'spcp8x5_usb_ctrl_arg'

'spcp8x5_usb_ctrl_arg' has been unused since the original
commit 619a6f1d ("USB: add usb-serial spcp8x5 driver").

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Johan Hovold <johan@kernel.org>