nfp: add nfp_reprs_get_locked() helper

The write side of repr tables is always done under pf->lock.
Add a helper to dereference repr table pointers under protection
of that lock.

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>