Strictly, dma_map_sg() may coalesce SG entries, but in practise on iMX
hardware, this will never happen.  However, dma_map_sg() can fail, and
we completely fail to check its return value.  So, fix this properly.

Arrange the code to map the scatterlist early, so we know how many
scatter table entries to allocate, and then fill them in.  This allows
us to keep relatively simple error cleanup paths.
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>