PCI/MSI: fix msi_mask() shift fix

Hidetoshi Seto points out that commit
bffac3c5 has wrong values in the array.
Rather than correct the array, we can just use a bounds check and
perform the calculation specified in the comment.  As a bonus, this will
not run off the end of the array if the device specifies an illegal
value in the MSI capability.
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

drivers/pci/msi.c

@@ -103,14 +103,12 @@ static void msix_set_enable(struct pci_dev *dev, int enable)
 	}
 }
 
-/*
- * Essentially, this is ((1 << (1 << x)) - 1), but without the
- * undefinedness of a << 32.
- */
 static inline __attribute_const__ u32 msi_mask(unsigned x)
 {
-	static const u32 mask[] = { 1, 2, 4, 0xf, 0xff, 0xffff, 0xffffffff };
-	return mask[x];
+	/* Don't shift by >= width of type */
+	if (x >= 5)
+		return 0xffffffff;
+	return (1 << (1 << x)) - 1;
 }
 
 static void msix_flush_writes(struct irq_desc *desc)