PCI: endpoint: pci-epf-vntb: Use array_index_nospec() on mws_size[] access

PCI: endpoint: pci-epf-vntb: Use array_index_nospec() on mws_size[] access

Follow common kernel idioms for indices derived from configfs attributes
and suppress Smatch warnings:

  epf_ntb_mw1_show() warn: potential spectre issue 'ntb->mws_size' [r]
  epf_ntb_mw1_store() warn: potential spectre issue 'ntb->mws_size' [w]

Also fix the error message for out-of-range MW indices and %lld format
for unsigned values.

Signed-off-by: Koichiro Den <den@valinux.co.jp>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20260105075606.1253697-1-den@valinux.co.jp