habanalabs: silence an uninitialized variable warning

Smatch warns that:

    drivers/misc/habanalabs/common/command_buffer.c:471 hl_cb_ioctl()
    error: uninitialized symbol 'device_va'.

Which is true, but harmless.  Anyway, it's easy to silence this by
adding a error check.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>