selftests: netdevsim: use suitable existing dummy file for flash test

The file name used in flash test was "dummy" because at the time test
was written, drivers were responsible for file request and as netdevsim
didn't do that, name was unused. However, the file load request is
now done in devlink code and therefore the file has to exist.
Use first random file from /lib/firmware for this purpose.

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