shell: kconfig: Fix wrong UART_SHELL_ON_DEV_NAME value due to typo

The $(dt_chosen_label) preprocessor function should be passed
$(DT_CHOSEN_Z_SHELL_UART), not $(UART_SHELL_ON_DEV_NAME).

DT_CHOSEN_Z_SHELL_UART is set right before the Kconfig symbol
definition, to work around the comma in 'zephyr,shell-uart'.

The Kconfig preprocessor functions are defined in
scripts/kconfig/kconfigfunctions.py.

Fixes: #19178

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>