serial: pl011: fix fall back from DMA to interrupt mode

When falling back from DMA to interrupt mode the receive interrupt has to
be re-enabled to catch new incoming data.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>