PCI/MSI: Cache Multiple Message Capable in struct msi_desc

The Multiple Message Capable field in the MSI Message Control register
indicates how many vectors the device supports.  This field is read-only,
so cache it in msi_desc to avoid reading it repeatedly.

Since we cache the extracted field (not the entire Message Control
register), we can use msi_mask() instead of msi_capable_mask(), which is
then unused, so remove it.

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