netdevsim: rename devlink.c to dev.c to contain per-dev(asic) items

The existing devlink.c code is going to be extended to represent asic
device on a bus. As this is about more than just devlink,
rename the file. Do appropriate prefix renaming alongside with that.

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