crypto: inside_secure - Avoid dma map if size is zero

From commit d03c5441 ("dma-mapping: disallow .map_sg
operations from returning zero on error"), dma_map_sg()
produces warning if size is 0. This results in visible
warnings if crypto length is zero.
To avoid that, we avoid calling dma_map_sg if size is zero.

Signed-off-by: Peter Harliman Liem <pliem@maxlinear.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>