smb1351-charger: Fix possible Null pointer dereference

The adc notification callback assumes that it will end up selecting
the correct state. However, if the parameters are outside the
expected values, the code could end up dereferencing a null pointer.

Fix this by returning before that pointer is dereferenced.

Change-Id: I42642b475a1032c3a7395336381f3bd6c1396dde
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>