sata_mv fix MSI irq race condition

Fix a (rare) race condition in mv_interrupt() when using MSI.

The value of hpriv->main_irq_mask_addr can change on on the fly, and without this patch we could end up writing back a stale copy to the hardware.

--

** This is required for 2.6.29 and beyond ***

@@ -2218,12 +2218,13 @@

		else
			handled = mv_host_intr(host, pending_irqs);
	}

- spin_unlock(&host->lock);

	/* for MSI: unmask; interrupt cause bits will retrigger now */
	if (using_msi)
		writel(hpriv->main_irq_mask, hpriv->main_irq_mask_addr);

+ spin_unlock(&host->lock);
+

	return IRQ_RETVAL(handled);
}

applied