IB/ipath: Workaround problem of errormask register being overwritten

On some system hardware, we are seeing moderately common cases of the
chip errormask register being overwritten due to a chip bug in iba6120
that is triggered by a vendor-specific PCIe broadcast message.  This
patch merely checks periodically, and corrects it if needed (the
overwrite can cause us to not get error and hardware error
interrupts).  Also, make dd->ipath_errormask the one, true canonical
source for kr_errormask, and remove references to ipath_ignorederrs as
it is currently unused.

Signed-off-by: Dave Olson <dave.olson@qlogic.com>
Signed-off-by: John Gregor <john.gregor@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>