crypto: atmel-tdes - Clarify how tdes dev gets allocated to the tfm

The tdes dev gets allocated to the tfm at alg->init time, there's no
need to overwrite the pointer to tdes_dd afterwards.
There's a single IP per SoC anyway, the first entry from the
atmel_tdes.dev_list is chosen without counting for tfms for example,
in case one thinks of an even distribution of tfms across the TDES
IPs: there's only one. At alg->init time the ctx->dd should already
be NULL, there's no need to check its value before requesting for a
tdes dev.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>