usb: gadget: uvc: fix NULL pointer dereference when usb hotplug

When do uvc hotplug test on RV1126 EVB, it may crash in the
uvc_v4l2_streamon() with the following error log. Because
it tries to enable the video stream after usb disconnect.

[ 1748.947755] configfs-gadget gadget: uvc_function_disable
[ 1748.947947] android_work: sent uevent USB_STATE=DISCONNECTED
[ 1748.955347] Unable to handle kernel NULL pointer dereference at virtual address 00000003
[ 1748.956158] pgd = ef2a7e72
[ 1748.956550] [00000003] *pgd=6dde7835
[ 1748.956893] Internal error: Oops: 17 [#1] PREEMPT SMP ARM
[ 1748.957381] Modules linked in: galcore(O) bcmdhd
[ 1748.957819] CPU: 3 PID: 2706 Comm: uvc_gadget_pthr Tainted: G        W  O      4.19.111 #9
[ 1748.958567] Hardware name: Generic DT based system
[ 1748.959218] PC is at uvcg_video_enable+0xb8/0x228
[ 1748.959775] LR is at vb2_core_streamon+0x11c/0x15c
......
[ 1749.041063] [<b056a2cc>] (uvcg_video_enable) from [<b0569968>] (uvc_v4l2_streamon+0x28/0x70)
[ 1749.041906] [<b0569968>] (uvc_v4l2_streamon) from [<b0590b54>] (__video_do_ioctl+0x1c8/0x3a0)
[ 1749.042681] [<b0590b54>] (__video_do_ioctl) from [<b0594288>] (video_usercopy+0x200/0x494)
[ 1749.043475] [<b0594288>] (video_usercopy) from [<b0220c38>] (do_vfs_ioctl+0xac/0x798)
[ 1749.044178] [<b0220c38>] (do_vfs_ioctl) from [<b0221358>] (ksys_ioctl+0x34/0x58)
[ 1749.044843] [<b0221358>] (ksys_ioctl) from [<b0101000>] (ret_fast_syscall+0x0/0x4c)

Signed-off-by: William Wu <william.wu@rock-chips.com>
Change-Id: I6bb58133aaade0ff389fa4af2cfc05fe598de250

Author: wuliangfeng

drivers/usb/gadget/function/uvc_v4l2.c

@@ -198,6 +198,9 @@ uvc_v4l2_streamon(struct file *file, void *fh, enum v4l2_buf_type type)
 	if (type != video->queue.queue.type)
 		return -EINVAL;
 
+	if (uvc->state != UVC_STATE_CONNECTED)
+		return -ENODEV;
+
 	/* Enable UVC video. */
 	ret = uvcg_video_enable(video, 1);
 	if (ret < 0)