MSI/powerpc: Use __read_msi_msg() instead of read_msi_msg()

rtas_setup_msi_irqs() already has the struct msi_desc pointer required by
__read_msi_msg(), so call it directly instead of having read_msi_msg() look
it up from the IRQ.

No functional change.

[bhelgaas: changelog]
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: linuxppc-dev@lists.ozlabs.org