tty: serial: ifx6x60: Use helper variable for dev

The &spi->dev is used so many times that the code gets
visibly better by introducing a simple dev helper variable.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20200311083131.693908-1-linus.walleij@linaro.org


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