IB/core: Init subsys if compiled to vmlinuz-core

Once infiniband is compiled as a core component its subsystem must be
enabled before device initialization. Otherwise there is a NULL pointer
dereference during mlx4_core init, calltrace:
->device_add
  if (dev->class) {
     deref  dev->class->p =>NULLPTR

#Config
CONFIG_NET_DEVLINK=y
CONFIG_MAY_USE_DEVLINK=y
CONFIG_MLX4_EN=y

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>