USB: serial: sierra driver zero length packet fix

- Fixed a problem with transferring packets with size a multiple of Bulk
  Xfer size in function sierra_write(). Added transfer flag
  URB_ZERO_PACKET before submitting the urb to trigger Zero-length data
  transfer when packet size is a multiple of Bulk Xfer.

Signed-off-by: Elina Pasheva <epasheva@sierrawireless.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>