ppp: use for_each_set_bit_from

Use for_each_set_bit_from to iterate over all the set bit in a memory
region.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Dmitry Kozlov <xeb@mail.ru>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>