IB/core: Remove pointer casting from void to net_device

This patch avoids unnecessary type casting from void to net_device.

CC: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>