drivers: serial: uart_nrfx_uarte: Patch RX init

The initialization of the UARTE is attempting to handle potential
improper handover from the bootloader. This handling is additional
complexity which should be fixed in the bootloader or whatever
component is failing to deinit the UART properly.

This commit removes the handling of improper handover from the
bootloader, while additionally clearing the ERROR flag when suspending
the UARTE component.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>