rpmsg: virtio: Free driver_override when rpmsg_remove()

Free driver_override when rpmsg_remove(), otherwise
the following memory leak will occur:

unreferenced object 0xffff0000d55d7080 (size 128):
  comm "kworker/u8:2", pid 56, jiffies 4294893188 (age 214.272s)
  hex dump (first 32 bytes):
    72 70 6d 73 67 5f 6e 73 00 00 00 00 00 00 00 00  rpmsg_ns........
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<000000009c94c9c1>] __kmem_cache_alloc_node+0x1f8/0x320
    [<000000002300d89b>] __kmalloc_node_track_caller+0x44/0x70
    [<00000000228a60c3>] kstrndup+0x4c/0x90
    [<0000000077158695>] driver_set_override+0xd0/0x164
    [<000000003e9c4ea5>] rpmsg_register_device_override+0x98/0x170
    [<000000001c0c89a8>] rpmsg_ns_register_device+0x24/0x30
    [<000000008bbf8fa2>] rpmsg_probe+0x2e0/0x3ec
    [<00000000e65a68df>] virtio_dev_probe+0x1c0/0x280
    [<00000000443331cc>] really_probe+0xbc/0x2dc
    [<00000000391064b1>] __driver_probe_device+0x78/0xe0
    [<00000000a...