spi/pl022: pass the returned sglen to the DMA engine

The sglen return by the dma_map_sg() should be passed to the DMA
engine, not the one passed in. If we one day have a DMA mapper
that can coalesce entries, this will bug due to a too large
number of entries being passed in.

Reported-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>