[SCSI] ipr: fix mailbox register definition and add a delay before reading

The definition for the mailbox register for new adapters was incorrect.  The
value has been updated to the correct offset.

After an adapter reset, the mailbox register on the new adapters takes a
number of seconds to stabilize.  A delay has been added before reading the
register.

Signed-off-by: Wayne Boyer <wayneb@linux.vnet.ibm.com>
Acked-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>