habanalabs: initialize variable to default value

Fix the following smatch error in unmap_device_va():
error: uninitialized symbol 'rc'.

Signed-off-by: Tomer Tayar <ttayar@habana.ai>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Link: https://lore.kernel.org/r/20200601065648.8775-1-oded.gabbay@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>