a mailbox command and which registers to copy back in when
the command completes, the bits being set need to not only
specify what bits you want to add from the default from the
table but also what bits you want *subtract* (mask) from the
default from the table.
A failing ISP2200 command pointed this out.
Much appreciation to: marius, who persisted and narrowed down what
the failure delta was, and shamed me into actually fixing it.
MFC after: 1 week
2bf9d2e61f