scsi: ibmvscsi: redo driver work thread to use enum action states

The current implemenation relies on two flags in the driver's private host
structure to signal the need for a host reset or to reenable the CRQ after
a LPAR migration. This patch does away with those flags and introduces a
single action flag and defined enums for the supported kthread work
actions. Lastly, the if/else logic is replaced with a switch statement.

Signed-off-by: Tyrel Datwyler <tyreld@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>