bnxt_en: Use SWITCHDEV_SET_OPS().

Suggested by Jakub Kicinski.

Fixes: c124a62f ("bnxt_en: add support for port_attr_get and and get_phys_port_name")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>