IB/core: Tidy ib_resolve_eth_dmac

No reason to call rdma_ah_retrieve_grh, tidy whitespace, and add a
function comment block.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>