usb: cdc-wdm: remove redundant assignment to rv

The variable rv is assigned with a value that is never read and
it is re-assigned a new value on the next statement. The
assignment is redundant and can be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>