net/tls: don't re-check msg decrypted status in tls_device_decrypted()

tls_device_decrypted() is only called from decrypt_skb_update(),
when ctx->decrypted == false, there is no need to re-check the bit.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>