nvmet: pci-epf: Do not uselessly write the CSTS register

nvmet: pci-epf: Do not uselessly write the CSTS register

The function nvmet_pci_epf_poll_cc_work() will do nothing if there are
no changes to the controller configuration (CC) register. However, even
for such case, this function still calls nvmet_update_cc() and uselessly
writes the CSTS register. Avoid this by simply rescheduling the poll_cc
work if the CC register has not changed.

Also reschedule the poll_cc work if the function
nvmet_pci_epf_enable_ctrl() fails to allow the host the chance to try
again enabling the controller.

While at it, since there is no point in trying to handle the CC register
as quickly as possible, change the poll_cc work scheduling interval to
10 ms (from 5ms), to avoid excessive read accesses to that register.

Fixes: 0faa0fe6f90e ("nvmet: New NVMe PCI endpoint function target driver")
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>