crypto: aspeed - fix type warnings

This patch fixes following warnings:

1. sparse: incorrect type in assignment (different base types)
Fix: change to __le32 type.
2. sparse: cast removes address space '__iomem' of expression
Fix: use readb to avoid dereferencing the memory.

Signed-off-by: Neal Liu <neal_liu@aspeedtech.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>