There are two modes for mailbox command (cmd) queue, i.e., event mode and
poll mode. For each mode, we use corresponding semaphores to protect the
cmd queue resource competition, so called event_sem and poll_sem. During
cmd init, both semaphores are initialized and poll mode is selected.
Thus, there is no need to up poll_sema again in cmd_use_polling.

Furthermore, there is no need to down the sema of the other side while
switching mode. This patch aims to decouple the switch between event mode
and poll mode of cmd.

Link: https://lore.kernel.org/r/1567068102-56919-2-git-send-email-liweihang@hisilicon.comSigned-off-by: Yixian Liu <liuyixian@huawei.com>
Signed-off-by: Weihang Li <liweihang@hisilicon.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>