crypto: aead - move aead_request_cast helper to aead.h

Move the helper function to common header for everybody to use.

changes in v2:
- move the helper to crypto/internal/aead.h
  instead of crypto/aead.h

Signed-off-by: Tadeusz Struk <tadeusz.struk@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>