pci_ep: fix wrong addressing to barno

barno was mistakely readed from the target structure,
resulting in undefined behavious depending on the previous memory
content. fix that.

Fixes: bb413337 ("pci_ep: add pci endpoint sandbox driver")
Signed-off-by: Ramon Fried <rfried.dev@gmail.com>
[trini: Drop unused bar_idx]
Signed-off-by: Tom Rini <trini@konsulko.com>