MFC r301293:
When negotiating NTB_SB01BASE_LOCKUP workaround, don't try to limit the
BAR size to 1MB. According to Xeon v3 specifications and my tests, that
size register is write-once and so not writeable after BIOS written it.

Instead of that, make the code work with BAR of any sufficient size,
properly calculating offset within its base. It also simplifies the code.

Sponsored by: iXsystems, Inc.