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: baecd7e5 drivers: uart_ns16550: Remove CMake-based templating
Signed-off-by: Timo Teräs <timo.teras@iki.fi>