drivers: uart_nrfx_uarte: Fix USE_LOW_POWER macro for PINCTRL case

This macro incorrectly uses DT_NODE_HAS_PROP() to check the truth value
of the "disable-rx" boolean property. In consequence, it always assumes
that RX is disabled and the low power mode needs to be used. Fix this
by replacing the check with DT_PROP().

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