crypto: ixp4xx - silence uninitialized variable warning

Smatch complains that "dma" is uninitialized if dma_pool_alloc() fails.
This is true, but also harmless.  Anyway, move the assignment after the
error checking to silence this warning.

Fixes: 586d492f ("crypto: ixp4xx - fix building wiht 64-bit dma_addr_t")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>