drivers: serial: nrfx_uarte: Fix bool endtx-stoptx prop check

The dt_nodelabel_bool_prop must be used to check for a boolean property,
not the dt_nodelabel_has_prop. Using the latter caused the
UART_X_ENHANCED_POLL_OUT condition to be not fulfilled, despite the
fact that the property was not set on nRF52840 and nRF91.

Signed-off-by: Rafał Kuźnia <rafal.kuznia@nordicsemi.no>