misc: pci_endpoint_test: Use -EINVAL for small subrange size

misc: pci_endpoint_test: Use -EINVAL for small subrange size

The sub_size check ensures that each subrange is large enough for 32-bit
accesses. Subranges smaller than sizeof(u32) do not satisfy this
assumption, so this is a local sanity check rather than a resource
exhaustion case.

Return -EINVAL instead of -ENOSPC for this case.

Suggested-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Koichiro Den <den@valinux.co.jp>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Link: https://patch.msgid.link/20260320140139.2415480-1-den@valinux.co.jp