scsi: lpfc: fix unused variable warning

The newly introduced 'cpu' variable is only used inside of an optional
block, so we get a warning without CONFIG_SCSI_LPFC_DEBUG_FS:

drivers/scsi/lpfc/lpfc_nvme.c: In function 'lpfc_nvme_io_cmd_wqe_cmpl':
drivers/scsi/lpfc/lpfc_nvme.c:968:30: error: unused variable 'cpu' [-Werror=unused-variable]
  uint32_t code, status, idx, cpu;

Move the declaration into the same block to avoid the warning.

Fixes: 63df6d63

 ("scsi: lpfc: Adapt cpucheck debugfs logic to Hardware Queues")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>