PCI: endpoint: pci-epf-vntb: Implement .db_vector_count()/mask() for doorbells

Implement .db_vector_count() and .db_vector_mask() so NTB core/clients can
map doorbell events to per-vector work and avoid the thundering-herd
behavior.

pci-epf-vntb reserves two slots in db_count: slot 0 for link events and
slot 1 which is historically unused. Therefore the number of doorbell
vectors is (db_count - 2).

Report vectors as 0..N-1 and return BIT_ULL(db_vector) for the
corresponding doorbell bit. Build db_valid_mask from a validated vector
count so out-of-range db_count values cannot create invalid shifts.

Signed-off-by: Koichiro Den <den@valinux.co.jp>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20260513024923.451765-8-den@valinux.co.jp

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