Input: synaptics - clear device info before filling in

synaptics_query_hardware() was being passed a 'struct synaptics_device_info'
in uninitialized stack memory, then not always initializing all fields.
This caused garbage to show up in certain fields, making the touchpad
unusable.

Fix by zeroing the device info, so all fields default to 0.

Fixes: 6c53694f ("Input: synaptics - split device info into a separate structure")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>