drivers: uart_native_posix: Fix reading file input
drivers: uart_native_posix: Fix reading file input

If the read function returns value of zero, there is no more data
in the file and the function should return value of -1.

Signed-off-by: Marek Pieta <d27fd21839ecf9c86fcf0dd8561035831899cc5a@nordicsemi.no>

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