Properly initialize device_data_t when downloading

The old gtk branch started out with device_data_t explicitly cleared,
but the Qt version never did that.  And we actually depend on the
deviceid in particular being initialized to zero (and then we fill in
the details in the divecomputer download callbacks)

Not properly initializing it meant that we ended up with random
deviceid's that got added to the divecomputer device lists, and then
saved to the XML file without actually matching the data in the dive
computers in the actual *dives*.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>