scsi: cxlflash: Access interrupt trigger page from xive directly

xive is already mapping the trigger page in kernel space and it can be
accessed through standard APIs, so let's reuse it and simplify the code.

Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Acked-by: Matthew R. Ochs <mrochs@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200403153838.29224-2-fbarrat@linux.ibm.com