IXP4xx: Fix ixp4xx_crypto little-endian operation.

Fixes the following on IXP425 little-endian:

NPE-C: firmware functionality 0x5, revision 0x2:1
alg: skcipher: Test 1 failed on encryption for ecb(des)-ixp4xx
00000000: 01 23 45 67 89 ab cd e7
alg: skcipher: Test 1 failed on encryption for ecb(des3_ede)-ixp4xx
00000000: 73 6f 6d 65 64 61 74 61
alg: skcipher: Test 1 failed on encryption for ecb(aes)-ixp4xx
00000000: 00 11 22 33 44 55 66 77 88 99 aa bb cc dd ee ff

Signed-off-by: Krzysztof Hałasa <khc@pm.waw.pl>
Acked-by: Christian Hohnstaedt <chohnstaedt@innominate.com>