usb-device-manager: Remove useless cast

spice_usb_device_new is already returning a SpiceUsbDevice pointer.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Snir Sheriber <ssheribe@redhat.com>
2 jobs for master in 4 minutes and 40 seconds (queued for 21 seconds)
Status Job ID Name Coverage
  Test
passed #452350
fedora

00:04:40

passed #452351
windows

00:02:51