Prevent nav mode from being -1 to avoid crash

Computing the nav mode would return -1 if there were no difference in
sys ui new and old bar mode. When dumping the data from nav bar, it
would crash if bar mode was -1 and there is no static constant for -1
bar mode. Therefore prevent setting -1 as a valid bar mode.

Test: manual
Change-Id: I9747f7aa7273b4bc05d26b08130818f059645187
Fixes: 117324620