USB: serial: metro-usb: drop redundant URB reinitialisation

No need to reinitialise the interrupt-in URB with values that have not
changed before (some) resubmissions.

This also allows the interrupt-in callback to have a single path for URB
resubmission.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>