TTY: amiserial, pass tty down to functions

This avoids pain with tty refcounting and touching tty_port in the
future. It allows us to remove some info->tty tests because the tty
passed down to them can never be NULL.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>