drivers: uart_nrfx_uarte: Fix RX auto disabling routine

This is a follow-up to commit 11bbdb03.

When RX is automatically disabled because all provided RX buffers have
been filled up, the rx_enabled flag needs to be cleared, otherwise it
will be impossible to enable RX again.

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