drivers: serial: Add error code to uart_rx_buf_rsp

API did not cover a case when uart_rx_buf_rsp is called when receiver
is already disabled. It may happen if uart_rx_buf_rsp is called too
late and active transfer is already finished. In that case -EACCES is
returned.

Some implementations already returned that error in that case.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>