get_new_macaddr: add 1 to the mac address instead of starting from 0x14

If someone have 2 adapters with the same hardware, there is a possibility
the first 5 bytes of their mac address to be the same.
I this case we will have mac address collision if we change both of them to
XX:XX:XX:XX:XX:14, one way to avoid this is to add 1 to their last byte.

Fix #47