[PPP]: Fixed hardware RX checksum handling

When we pull the PPP protocol off the skb, we forgot to update the
hardware RX checksum.  This may lead to messages such as

	dsl0: hw csum failure.

Similarly, we need to clear the hardware checksum flag when we use
the existing packet to store the decompressed result.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>