bnxt_en: Refactor __bnxt_vf_reps_destroy().

Add a new helper function __bnxt_free_one_vf_rep() to free one VF rep.
We also reintialize the VF rep fields to proper initial values so that
the function can be used without freeing the VF rep data structure.  This
will be used in subsequent patches to free and recreate VF reps after
error recovery.

Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Reviewed-by: Sriharsha Basavapatna <sriharsha.basavapatna@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>