IB/cxgb3: fix misspelling in header guard

Use CXGB3_... instead of CXBG3_...

Fixes: a85fb338 ("IB/cxgb3: Move user vendor structures")
Cc: stable@vger.kernel.org # 4.9
Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Acked-by: Steve Wise <swise@chelsio.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>