drivers: Report correct errno in uart_native_posix driver
drivers: Report correct errno in uart_native_posix driver

This driver carefully saved the errno value from the failing call and
then didn't use it in the ERROR report, using the potentially invalid
current errno value (which may have been set by the close call).

Signed-off-by: Keith Packard <fd7f967895e9f35e58ec8a62a847a54d7fa7275f@keithp.com>

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