misc: ti-st: don't check for tty data == NULL

tty data passed to tty_ldisc_ops::receive_buf() are never NULL. Remove
this check.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230731080244.2698-11-jirislaby@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>