crypto: omap-aes-gcm - check length of assocdata in RFC4106 mode

RFC4106 requires the associated data to be a certain size, so reject
inputs that are wrong. This also prevents crashes or other problems due
to assoclen becoming negative after subtracting 8 bytes.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Tero Kristo <t-kristo@ti.com>
Tested-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>