USB: serial: sierra driver bug fix for composite interface

This patch fixes a problem in sierra_send_setup() function when
composite devices are used. One should not be sending ACM commands to
interfaces that are OBEX. Doing this causes an apparent failure as the
ACM command has to time out before the interface can start being used.

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