drivers: uart_ns16550: Fix dts hw_flow_control mapping to config
drivers: uart_ns16550: Fix dts hw_flow_control mapping to config

DT_INST_NODE_HAS_PROP() returns true always since the boolean
tag is valid. Use DT_INST_PROP_OR() to get the real value.

Fixes: baecd7e55a6 drivers: uart_ns16550: Remove CMake-based templating
Signed-off-by: Timo Teräs <266f729f1f620250264df63d5dfd302c2a88080e@iki.fi>

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