drivers: uart: Fix incorrect len and offset in nrf UARTE async API.

Due to longer than expected user callback handling, rx byte counting
got out of sync with real values. It leads to incorrect values
reported to user. This fix adds sync point at the end of buffer.

When using hardware rx counting this issue should not occur.

Signed-off-by: Mieszko Mierunski <mieszko.mierunski@nordicsemi.no>