PCI: endpoint: Fix a Kconfig prompt of vNTB driver

vNTB driver and NTB driver have same Kconfig prompt. Changed to make it
distinguishable.

Link: https://lore.kernel.org/r/20230202103832.2038286-1-mie@igel.co.jp
Fixes: e35f56bb

 ("PCI: endpoint: Support NTB transfer between RC and EP")
Signed-off-by: Shunsuke Mie <mie@igel.co.jp>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>