PCI: Set ->mask_pos correctly

The "+" operation has higher precedence than "?:" and ->msi_cap is
always non-zero here so the original statement is equivalent to:

    entry->mask_pos = PCI_MSI_MASK_64;

Which wasn't the intent.

[bhelgaas: my fault from 78b5a310]
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>