pci/msix: Fix msix pba read vector poll end calculation

pci/msix: Fix msix pba read vector poll end calculation

The end vector calculation has a bug that results in polling fewer
than required vectors when reading at a non-zero offset in PBA memory.

Fixes: bbef882cc193 ("msi: add API to get notified about pending bit poll")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-Id: <20241212120402.1475053-1-npiggin@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 42e2a7a0ab23784e44fcb18369e06067abc89305)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>