USB: serial: remove unnecessary reinitialisations of urb->dev

Remove unnecessary reinitialisations of urb->dev before each submission,
which were based on the (no longer valid) assumption that serial->dev
will be set to NULL on close.

Compile-only tested.

Cc: Matthias Bruestle and Harald Welte <support@reiner-sct.com>
Cc: Lonnie Mendez <dignome@gmail.com>
Cc: Peter Berger <pberger@brimson.com>
Cc: Al Borchers <alborchers@steinerpoint.com>
Cc: Support Department <support@connecttech.com>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>