bnxt_en: fix clearing devlink ptr from bnxt struct

The routine bnxt_link_bp_to_dl() is used to set the devlink ptr
in bnxt struct (bp) and also to set the bnxt back ptr in
the devlink struct.  If devlink_register() fails, bp->dl must
be cleared which is not happening currently. This patch fixes
bnxt_link_bp_to_dl() to clear bp->dl by passing  a NULL dl ptr.

Fixes: 4ab0c6a8 ("bnxt_en: add support to enable VF-representors")
Signed-off-by: Sathya Perla <sathya.perla@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>