PCI express device status register has W1C feature. Writing 0 has
no effect. Make sure to clear error bits by writing 1. [1]
While I'm here use predefined value instead of hardcodig magic
vlaue.

Submitted by:	msaitoh at NetBSD [1]

Move device specific flag configuration to attach routine.
The softc obtained in device probe wouldn't be the same one used in
device attach. Drivers should not assume any values stored in softc
structure in probe routine will be available for its attach routine.