PCI: endpoint: pci-epf-vntb: Align MW naming with config names

The config file related to the memory windows start the numbering of
the MW from 1. The other NTB function does the same, yet the enumeration
defining the BARs of the vNTB function starts numbering the MW from 0.

Both numbering should be fine, but mixing the two is a bit confusing. The
configfs file being the interface with userspace, keep that stable and
consistently start the numbering of the MW from 1.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
[mani: commit message rewording]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20250603-pci-vntb-bar-mapping-v2-2-fc685a22ad28@baylibre.com