scsi: lpfc: Fix crash when cpu count is 1 and null irq affinity mask

When a configurations runs with a single cpu (such as a kdump kernel),
which causes the driver to request a single vector, when the driver
subsequently requests an irq affinity mask, the mask comes back null.  The
driver currently does nothing in this scenario, which leaves mappings to
hardware queues incomplete and crashes the system.

Fix by recognizing the null mask and assigning the vector to the first cpu
in the system.

Signed-off-by: Dick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: James Smart <jsmart2021@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>