nvme: change return type of nvme_poll_cq() to bool

nvme: change return type of nvme_poll_cq() to bool

The nvme_poll_cq() function currently returns the number of CQEs
found, However, only one caller, nvme_poll(), requires a boolean
value to check whether any CQE was completed. The other callers do
not use the return value at all.

To better reflect its usage, update the return type of nvme_poll_cq()
from int to bool.

Signed-off-by: Yongsoo Joo <ysjoo@kookmin.ac.kr>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>