pci: add accessors to get/set registers by mask

pci_regs.h specifies many registers by mask +
shifted register values.
There's always some duplication when using such:
for example to override device type, we would need:

pci_word_test_and_clear_mask(cap + PCI_EXP_FLAGS,
     PCI_EXP_FLAGS_TYPE);
pci_word_test_and_set_mask(cap + PCI_EXP_FLAGS,
    PCI_EXP_TYPE_ENDPOINT << (ffs(PCI_EXP_FLAGS_TYPE) - 1));

Getting such registers also uses some duplication:

word = pci_get_word(cap + PCI_EXP_FLAGS) & PCI_EXP_FLAGS_TYPE;
if ((word >> ffs((PCI_EXP_FLAGS_TYPE) - 1)) == PCI_EXP_TYPE_ENDPOINT)

Add API to access such registers in one line:
pci_set_word_by_mask(cap + PCI_EXP_FLAGS, PCI_EXP_FLAGS_TYPE,
		     PCI_EXP_TYPE_ENDPOINT)

and
word = pci_get_word_by_mask(cap + PCI_EXP_FLAGS, PCI_EXP_FLAGS_TYPE)
if (word == PCI_EXP_TYPE_ENDPOINT)

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
