USB: serial: sierra: put reset_resume callback back.

A few patches ago, I removed the reset_resume callback in this driver.
Now that the usb-serial core supports reset_resume, put this driver
callback back as well, so it should work identically to how it was
originally.

Now if this function really is doing what it should be doing, well,
that's a different story, but we are at least doing the identical thing
that we were before...

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>