mISDN: hfcpci: Test both vendor & device ID for Digium HFC4S

The device ID alone does not uniquely identify a device.  Test both the
vendor and device ID to make sure we don't mistakenly think some other
vendor's 0xB410 device is a Digium HFC4S.  Also, instead of the bare hex
ID, use the same constant (PCI_DEVICE_ID_DIGIUM_HFC4S) used in the device
ID table.

No functional change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>