mkregtable: Fix sscanf handling

If you feed the tool a suitable bogus register map you can break it
in arbitary (code executing) ways. While this isn't a particularly
exciting or probable attack vector we still ought to fix it.

One of a set of sscanf issues reported by Jackie Chang

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>