ppp: use the correct function to check if a netdev name is in use

A new helper to detect if a net device name is in use was added. Use it
here as the return reference from __dev_get_by_name was discarded.

Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>