RDMA/siw: Mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases where
we are expecting to fall through.

This patch fixes the following warnings:

drivers/infiniband/sw/siw/siw_qp_rx.c: In function ‘siw_rdmap_complete’:
drivers/infiniband/sw/siw/siw_qp_rx.c:1214:18: warning: this statement may fall through [-Wimplicit-fallthrough=]
   wqe->rqe.flags |= SIW_WQE_SOLICITED;
   ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
drivers/infiniband/sw/siw/siw_qp_rx.c:1215:2: note: here
  case RDMAP_SEND:
  ^~~~

drivers/infiniband/sw/siw/siw_qp_tx.c: In function ‘siw_qp_sq_process’:
drivers/infiniband/sw/siw/siw_qp_tx.c:1044:4: warning: this statement may fall through [-Wimplicit-fallthrough=]
    siw_wqe_put_mem(wqe, tx_type);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/infiniband/sw/siw/siw_qp_tx.c:1045:3: note: here
   case SIW_OP_INVAL_STAG:
   ^~~~
drivers/infiniband/sw/siw/siw_qp_tx.c:1128:4: warning: this statement may fall through [-Wimplicit-fallthrough=]
    siw_wqe_put_mem(wqe, tx_type);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/infiniband/sw/siw/siw_qp_tx.c:1129:3: note: here
   case SIW_OP_INVAL_STAG:
   ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Bernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>