crypto: omap-aes-gcm - fix corner case with only auth data

Fix a corner case where only authdata is generated, without any provided
assocdata / cryptdata. Passing the empty scatterlists to OMAP AES core driver
in this case would confuse it, failing to map DMAs.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>