crypto: essiv - fix AEAD capitalization and preposition use in help text

"AEAD" is capitalized everywhere else.
Use "an" when followed by a written or spoken vowel.

Fixes: be1eb7f7 ("crypto: essiv - create wrapper template for ESSIV generation")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>