pci_set_mwi() has 'warn_unused_result' so the result should be checked.

Fixes the following W=1 kernel build warning(s):

 drivers/scsi/qla4xxx/ql4_init.c: In function ‘qla4xxx_pci_config’:
 drivers/scsi/qla4xxx/ql4_init.c:664:6: warning: variable ‘status’ set but not used [-Wunused-but-set-variable]

And if 'status' is removed:

 drivers/scsi/qla4xxx/ql4_init.c: In function ‘qla4xxx_pci_config’:
 drivers/scsi/qla4xxx/ql4_init.c:666:2: warning: ignoring return value of ‘pci_set_mwi’, declared with attribute warn_unused_result [-Wunused-result]
 666 | pci_set_mwi(ha->pdev);
 | ^~~~~~~~~~~~~~~~~~~~~

Link: https://lore.kernel.org/r/20200721164148.2617584-14-lee.jones@linaro.org
Cc: QLogic-Storage-Upstream@qlogic.com
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>