crypto: marvell/cesa - Silence gcc format-truncation false positive warnings

The heuristics used by gcc triggers false positive truncation
warnings in hifn_alg_alloc.  The warnings are false positives
because nengines is at most 2.

Make the buffer bigger and change the snprintf to use unsigned
integers to eliminate these warnings.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>