netdevsim: generate random switch id instead of using dev id

Current implementation of parent_id/switch_id does not follow the
original idea of being unique. The values are "0", "1", etc. Instead of
that, generate 32 random bytes.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>