[SCSI] ibmvscsi: Don't fail EH due to insufficient resources

The ibmvscsi driver currently has a bug in it which can result
in it using up all its event structs for commands. If something
results in all those commands timing out, we won't have any resources
left to send aborts or resets. This results in escalating to a host reset
in order to recover, which is a bit heavy handed. This fixes it
by reducing can_queue by two in order to have resources to do EH.
It also changes the max_requests module parameter so that it is not
writable at runtime, since the code really does not handle it changing
at runtime.

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