USB: ssu100: set tty_flags in ssu100_process_packet

flag was never set in ssu100_process_packet.  Add logic to set it
before calling tty_insert_flip_*

Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>