[PPP]: Remove ptr comparisons to 0

fix sparse warnings "Using plain integer as NULL pointer"

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc drivers/net/ppp_async.c
Simple merge

diff --cc drivers/net/ppp_generic.c
Simple merge

diff --cc drivers/net/ppp_synctty.c
Simple merge