drivers: uart_nrfx_uart: Fix incorrect uses of DT_NODE_HAS_PROP

Values of boolean DT properties need to be checked with DT_PROP(),
not DT_NODE_HAS_PROP(). Fix two such incorrect calls in the nRF UART
driver.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>