selftests: netdevsim: be less selective for FW for the devlink test

Commit 6151ff9c

 ("selftests: netdevsim: use suitable existing dummy
file for flash test") introduced a nice trick to the devlink flashing
test. Instead of user having to create a file under /lib/firmware
we just pick the first one that already exists.

Sadly, in AWS Linux there are no files directly under /lib/firmware,
only in subdirectories. Don't limit the search to -maxdepth 1.
We can use the %P print format to get the correct path for files
inside subdirectories:

$ find /lib/firmware -type f -printf '%P\n' | head -1
intel-ucode/06-1a-05

The full path is /lib/firmware/intel-ucode/06-1a-05

This works in GNU find, busybox doesn't have printf at all,
so we're not making it worse.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240224050658.930272-1-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>