pcib: Clear any pending slot status events during attach

Any events left over from boot firmware, etc. are not meaningful so
clear pending events during attach.  If they are still pending they
should remain asserted when the slot status register is re-read.

Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D49950

(cherry picked from commit ff45e47)

Author: bsdjhb

sys/dev/pci/pci_pci.c

@@ -1434,6 +1434,7 @@ pcib_setup_hotplug(struct pcib_softc *sc)
 
 	/* Clear any events previously pending. */
 	pcie_write_config(dev, PCIER_SLOT_STA, sc->pcie_slot_sta, 2);
+	sc->pcie_slot_sta = pcie_read_config(dev, PCIER_SLOT_STA, 2);
 
 	/* Enable HotPlug events. */
 	mask = PCIEM_SLOT_CTL_DLLSCE | PCIEM_SLOT_CTL_HPIE |