gpio: exar: Fix iomap request

The UART driver already maps the resource for us. Trying to do this here
only fails and leaves us with a non-working device.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>