sf: Fix sf read for memory-mapped SPI flashes

Missing return after memcpy is done for memory-mapped SPI flashes,
hence added retun 0 after memcpy done.

The return is missing in below patch
"sf: Enable FDT-based configuration and memory mapping"
(sha1: bb8215f4)

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
Acked-by: Simon Glass <sjg@chromium.org>