PCI/MSI: Remove "pos" from the struct msi_desc msi_attrib

"msi_attrib.pos" is only used for MSI (not MSI-X), and we already cache the
MSI capability offset in "dev->msi_cap".

Remove "pos" from the struct msi_attrib and use "dev->msi_cap" directly.

[bhelgaas: changelog, fix whitespace]
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>