ppp: Fix wrong lcp_state handling on peer reconnection

[lzungri]

If a PPP peer disconnects and then tries to reconnect it will send an 'LCP configure request' packet. The code that handles that scenario seems to be clearing the wrong lcp_state flag (LCP_RX_UP instead of LCP_TX_UP) and thus the nuttx ppp client will keep sending IPCP packets which are rightfully dropped by the new peer since it is still in the LCP negotiation phase.

[cederom]

• Thank you @lzungri very nice catch :-)
• You have tested the patch and it fixes the issue in practice with various different setups? Please add short testing / verification description / confirmation.
• Comment needs reformat to pass CI, see https://nuttx.apache.org/docs/latest/contributing/coding_style.html.

[lzungri]

You have tested the patch and it fixes the issue in practice with various different setups? Please add short testing / verification description / confirmation.

@cederom I have a custom version of the pppd code from nuttx so I have only tested it in my specific setup, which doesn't involve running it under nuttx OS.
My testing involved connecting a linux pppd daemon to my custom nuttx code and confirming that the connection is re-established smoothly after restarting the linux pppd process.