net: thunder: make tx software timestamp independent

net: thunder: make tx software timestamp independent

skb_tx_timestamp() is used for tx software timestamp enabled by
SOF_TIMESTAMPING_TX_SOFTWARE while SKBTX_HW_TSTAMP is used for
SOF_TIMESTAMPING_TX_HARDWARE. As it clearly shows they are different
timestamps in two dimensions, it's not appropriate to group these two
together in the if-statement.

This patch completes three things:
1. make the software one standalone. Users are able to set both
timestamps together with SOF_TIMESTAMPING_OPT_TX_SWHW flag.
2. make the software one generated after the hardware timestamp logic to
avoid generating sw and hw timestamps at one time without
SOF_TIMESTAMPING_OPT_TX_SWHW being set.
3. move the software timestamp call as close to the door bell.

Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250508034433.14408-1-kerneljasonxing@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>