[SCSI] ibmvfc: Fixup module parms attributes

The ibmvfc driver really does not handle dynamically changing disc_threads.
To change this dynamically would cause confusion in the driver regarding
the number of event structs allocated. Fix this by simply not allowing
disc_threads to be changed at runtime.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>