USB: mos7840: remove unused variable

Fix warning about unused variable introduced by commit e681b66f2e19fa
("USB: mos7840: remove invalid disconnect handling") upstream.

A subsequent fix which removed the disconnect function got rid of the
warning but that one was only backported to v3.6.

Reported-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>