tls: rx: make tls_wait_data() return an recvmsg retcode

tls_wait_data() sets the return code as an output parameter
and always returns ctx->recv_pkt on success.

Return the error code directly and let the caller read the skb
from the context. Use positive return code to indicate ctx->recv_pkt
is ready.

While touching the definition of the function rename it.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>