USB: serial: sierra: remove reset_resume callback

This really just is the resume callback for the device, so use that,
especially as the usb-serial core just overrode this callback so it
wasn't being made anyway.

Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Johan Hovold <jhovold@gmail.com>
Cc: Anton Samokhvalov <pg83@yandex.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>