tty: synclink_gt: don't allocate and pass dummy flags

In synclinc_gt, the flag_buf is allocated, zeroed and passed to ldisc's
receive_buf(). It is never written to, so it serves as a dummy buffer.
That's unneeded because all ldiscs accept NULL as flags. That NULL
resolves to the TTY_NORMAL flag.

So drop all this nonsense.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230420093530.13133-1-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>