net: l2: ppp: Explicitly negotiate ACCM

Many cellular modems attempt to negotiate an ACCM value of 0x00000000.
While the PPP driver rejects this by default, it does not propose an
alternative. As a result, some modems default to using 0x00000000 after
LCP negotiation. Because the PPP driver expects all control characters
to be escaped, this causes issues during decoding. This change
negotiates an ACCM value of 0xffffffff to ensure compatibility with such
modems.

Signed-off-by: Vytautas Virvičius <vytautas@virvicius.dev>