subsys: modem: backends: uart: modem.backends.uart.async fails

[katgiadla]

Describe the bug
The test modem.backends.uart.async fails.

Observed for nrf5340dk_nrf5340_cpuapp.

To Reproduce
Steps to reproduce the behavior:

1. have nrf5340dk_nrf5340_cpuapp connected
2. connect pins follow tests/subsys/modem/backends/uart/boards/nrf5340dk_nrf5340_cpuapp.overlay
3. go to your zephyr dir
4. call $ZEPHYR_BASE/scripts/twister -vv --inline-logs --fixture gpio_loopback -T .tests/subsys/modem/backends/uart/ --device-testing --hardware-map map.yml --platform nrf5340dk_nrf5340_cpuapp -j 1
5. See handler.log and console output

Expected behavior
Valid console output

Impact
Not clear

Logs and console output

*** Booting Zephyr OS build zephyr-v3.5.0-4395-ga1d33f62fff9 ***
Running TESTSUITE modem_backend_uart_suite
===================================================================
START - test_transmit_receive
TX: 2,2
TX ACC: 2
RX: 2
TX: 8,8
TX ACC: 10

Environment (please complete the following information):

• OS: Ubuntu 20.04.5 LTS
• Toolchain Zephyr 0.16.1
• Commit SHA or Version used: zephyr-v3.5.0-4395-ga1d33f62fff9

Additional context
Used pin set for nrf5340dk_nrf5340_cpuapp:

uart1_default_alt: uart1_default_alt {
		group1 {
			psels = <NRF_PSEL(UART_TX, 0, 4)>,
				<NRF_PSEL(UART_RX, 0, 5)>;
		};
	};

[katgiadla]

Issue has not occurred again.