crypto: chcr - Fix error handling related to 'chcr_alloc_shash'

Up to now, 'crypto_alloc_shash()' may return a valid pointer, an error
pointer or NULL (in case of invalid parameter)
Update it to always return an error pointer in case of error. It now
returns ERR_PTR(-EINVAL) instead of NULL in case of invalid parameter.

This simplifies error handling.

Also fix a crash in 'chcr_authenc_setkey()' if 'chcr_alloc_shash()'
returns an error pointer and the "goto out" path is taken.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>