nfp: print a warning when binding VFs to PF driver

Users sometimes mistakenly try to manually bind the PF driver
to the VFs, print a warning message in that case.

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>