gatt: Fix double free

When todo->data is passed to a function and the function returns
success, we should always set todo->data to NULL to mark that this sg
ownership has been transferred to the called function.

BUG=chromium:805112
TEST=Tested with HID over GATT write report.

Change-Id: Icfffe4ea70d864fb03935d0f8d6977c6f4512e8d
1 file changed