USB: serial: cypress_m8: clean up initial-termios handling

Now that init_termios() is only called on first use, we can clean up the
cypress_m8 initial-termios handling.

Note that only the earthmate chip type used settings different from the
defaults provided by USB serial core, and that the chip type is indeed
known when init_termios is called at tty-install time.

Signed-off-by: Johan Hovold <johan@kernel.org>