nvme-core: fix comment to reflect right functions

The functions and the attribute listed in the comment doesn't exists in
the code, (ns->logging_enabled, nvme_passthru_err_log_enabled_store()
and nvme_passthru_err_log_enabled_show())

Update the comment with right function names and a comment
ns->head->passthru_err_log_enabled,
nvme_io_passthru_err_log_enabled_store() and
nvme_io_passthru_err_log_enabled_show().

Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Alan Adamson <alan.adamson@oracle.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>