serial: omap: Disallow RS-485 if rts-gpio is not specified

serial: omap: Disallow RS-485 if rts-gpio is not specified

The serial-omap driver requires an rts-gpio for RS-485 to work.
Historically it has allowed enabling RS-485 even if no rts-gpio was
specified in the device tree.

That doesn't make any sense, so disable RS-485 on probe if rts-gpio is
missing and disallow user space from enabling it.

Three NULL pointer checks for up->rts_gpiod can be dropped as a result,
simplifying the driver slightly.

Cc: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: https://lore.kernel.org/r/f191dcca0d8ea03598c463fc0d3fba8941ff2275.1662888075.git.lukas@wunner.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>