TTY: synclink_cs, fix build

Commit "TTY: synclink_cs, use dynamic tty devices" added a call to
tty_port_register_device with a proper device as the last argument.
But it was not correct and it causes build failures:
synclink_cs.c: In function ‘mgslpc_add_device’:
synclink_cs.c:2735:16: error: request for member ‘dev’ in something not a structure or union

info->p_dev is a pointer, so act as that.

I wonder why my build scripts did not notice. I have to re-check them.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>