4.2 disk driver bug (and fix!)

[Eugene Cristofor]

There is a bug in the 4.2 disk driver: bad sector forwarding
does not work for EAGLEs with System Industries controller.
Problem is that the SI controller clobbers the MBA registers -
one must use the drive registers `hpdc' and `hpda' to find
where the BSE error has occurred.

A fix is available - all changes are in vaxmba/hp.c

gene cristofor
vax135!gc