Kill ignore regshft flag

This flag was introduced to ignore bad firmware values. These were
present in older versions of EDK-II that many devices (both Ampere and
Amazon) used. QEMU also used this value, but fixed it. But since it's
tied to the firmware bug not the device name ID it doesn't make sense to
have it flagged there.

Sponsored by: Netflix
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D47947