drivers: net: ppp: Set point-to-point interface flag

Set the point-to-point flag because PPP is a point-to-point protocol. This
flag enables specific IPv6 neighbor discovery handling, to select the
correct sending interface. Without it set, a different interface might be
selected based on the destination address.

Signed-off-by: Marc Lasch <marc.lasch@husqvarnagroup.com>