crypto: Fix the logic to calculate hash with authattributes set

RFC 2315 Section 9.3 describes the message digesting process. The
digest calculated depends on whether the authenticated attributes are
present. In case of a scenario where the authenticated attributes are
present, the message digest that gets signed and is part of the pkcs7
message is computed from the auth attributes rather than the contents
field.

Check if the auth attributes are present, and if set, use the auth
attributes to compute the hash that would be compared with the
encrypted hash on the pkcs7 message.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>