crypto: arm64/crct10dif - Remove remaining 64x64 PMULL fallback code

The only remaining user of the fallback implementation of 64x64
polynomial multiplication using 8x8 PMULL instructions is the final
reduction from a 16 byte vector to a 16-bit CRC.

The fallback code is complicated and messy, and this reduction has
little impact on the overall performance, so instead, let's calculate
the final CRC by passing the 16 byte vector to the generic CRC-T10DIF
implementation when running the fallback version.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>