net/tls: make sure tls offload sets salt_size

Recent changes made to remove AES constants started using protocol
aware salt_size. ctx->prot_info's salt_size is filled in tls sw case,
but not in tls offload mode, and was working so far because of the
hard coded value was used.

Fixes: 6942a284

 ("net/tls: make inline helpers protocol-aware")
Signed-off-by: Rohit Maheshwari <rohitm@chelsio.com>
Link: https://lore.kernel.org/r/20201201090752.27355-1-rohitm@chelsio.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>