serial: sh-sci: Drop superfluous assignment in sci_request_dma()

No later code uses the assigned value, so it can be dropped.

Reported-by: Simon Horman <horms@verge.net.au>
Fixes: 2c4ee235 ("serial: sh-sci: Postpone DMA release when falling back to PIO")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>