PCI: endpoint: pci-epf-vntb: Reject unusable doorbell counts

PCI: endpoint: pci-epf-vntb: Reject unusable doorbell counts

pci-epf-vntb reserves slot 0 for link events and keeps slot 1 unused for
legacy layout compatibility. A db_count smaller than MIN_DB_COUNT leaves
no usable doorbell slot after those reservations.

Reject such configurations when configuring interrupts.

While at it, move MAX_DB_COUNT next to MIN_DB_COUNT. They are used as a
pair in the range check, and keeping them together makes the valid doorbell
range easier to read.

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/20260513024923.451765-5-den@valinux.co.jp