serial: pl011: (cosmetic) remove superfluous register write

In pl011_rx_chars() if pl011_dma_rx_trigger_dma() succeeds it will disable
the receive interrupt, no need to do this again.

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