bnxt_en: rename some xdp functions

Renaming bnxt_xmit_xdp to __bnxt_xmit_xdp to get ready for XDP_REDIRECT
support and reduce confusion/namespace collision.

Signed-off-by: Andy Gospodarek <gospo@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>