usb-serial: fix termios initialization logic

This patch (as1288) fixes the initialization logic in
serial_install().  A new tty always needs to have a termios
initialized no matter what, not just in the case where the lower
driver will override the termios settings.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>