ipr: Re-enable write same

Re-enable write same support for ipr RAID adapters.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Reviewed-by: Wen Xiong <wenxiong@linux.vnet.ibm.com>
Reviewed-by: Daniel Kreling <kreling@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <JBottomley@Odin.com>