google_dual_batt_gauge: exit dual battery work if one of battery not ready

Skip running dual battery worker for only one battery devices (another
one missing or earlier stage). Null psy causes panic in these cases.

Bug: 188768844
Signed-off-by: Jenny Ho <hsiufangho@google.com>
Change-Id: I9ca1d6d8412e4d10fb5724830092151b12487d96