Fix 'enum rdma_driver_id' to preserve other driver values before that
RDMA_DRIVER_CXGB3 was deleted. As this value is UAPI we can't affect
other values as of a deletion of one driver id.

Fixes: 30e0f6cf ("RDMA/iw_cxgb3: Remove the iw_cxgb3 module from kernel")
Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Link: https://lore.kernel.org/r/20191015075419.18185-2-leon@kernel.orgSigned-off-by: Doug Ledford <dledford@redhat.com>