drivers: uart_nrfx_uarte: Fix RX auto disabling routine
drivers: uart_nrfx_uarte: Fix RX auto disabling routine

This is a follow-up to commit 11bbdb030def3b4a4b97ee380b1091913e61d371.

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 <6788b8d556a8a447102338fc996c42180ecff44e@nordicsemi.no>

finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr