drivers: serial: uart_sam0: Fix build error with CONFIG_UART_ASYNC_API
drivers: serial: uart_sam0: Fix build error with CONFIG_UART_ASYNC_API

When we build this driver with CONFIG_UART_ASYNC_API enabled we get the
following build error:

uart_sam0.c: In function 'uart_sam0_init':
uart_sam0.c:558:35: error: redefinition of 'dev_data'
  558 |  struct uart_sam0_dev_data *const dev_data = DEV_DATA(dev);
uart_sam0.c:498:35: note: previous definition of 'dev_data' was here
  498 |  struct uart_sam0_dev_data *const dev_data = DEV_DATA(dev);

Fix this be removnig the duplicate at line 558.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr