nfp: populate bus-info on representors

We used to leave bus-info in ethtool driver info empty for
representors in case multi-PCIe-to-single-host cards make
the association between PCIe device and NFP many to one.
It seems these attempts are futile, we need to link the
representors to one PCIe device in sysfs to get consistent
naming, plus devlink uses one PCIe as a handle, anyway.
The multi-PCIe-to-single-system support won't be clean,
if it ever comes.

Turns out some user space (RHEL tests) likes to read bus-info
so just populate it.

While at it remove unnecessary app NULL-check, representors
are spawned by an app, so it must exist.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>