staging: r8188eu: interface type is always usb

This driver only supports chips that connect via usb. Remove
interface_type from struct adapter, use a constant in the one
place where the interface type is needed.

Acked-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20211005200821.19783-8-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>