Fix some logic in PCIe HotPlug; display EI status

The interpretation of the Electromechanical Interlock Status was
inverted, so we disengaged the EI if a card was inserted.
Fix it to engage the EI if a card is inserted.

When displaying the slot capabilites/status with pciconf:

• We inverted the sense of the Power Controller Control bit, saying the power was off when it was really on (according to this bit). Fix that.

• Display the status of the Electromechanical Interlock: EI(engaged) EI(disengaged)

Reviewed by: jhb
MFC after: 3 days
Sponsored by: Dell Inc.
Differential Revision: https://reviews.freebsd.org/D7426