net: ppp: Remove unnecessary (void*) conversions

No need cast (void*) to (struct sock *).

Signed-off-by: Wu Yunchuan <yunchuan@nfschina.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Link: https://lore.kernel.org/r/20230717031115.54432-1-yunchuan@nfschina.com


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