Both lan78xx and smsc75xx have a 'usb_context'
struct which is unused, since their original commits.

Remove them.
Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Link: https://lore.kernel.org/r/20240526205922.176578-1-linux@treblig.orgSigned-off-by: Paolo Abeni <pabeni@redhat.com>