[media] exynos-gsc: unregister video device node on driver removal

[media] exynos-gsc: unregister video device node on driver removal

The driver doesn't unregister the video device node when the driver is
removed, this keeps video device nodes that makes the machine to crash
with a NULL pointer dereference when nodes are attempted to be opened:

[   36.530006] Unable to handle kernel paging request at virtual address bf1f8200
[   36.535985] pgd = edbbc000
[   36.538486] [bf1f8200] *pgd=6d99a811, *pte=00000000, *ppte=00000000
[   36.544727] Internal error: Oops: 7 [#1] PREEMPT SMP ARM
[   36.550016] Modules linked in: s5p_jpeg s5p_mfc v4l2_mem2mem videobuf2_dma_contig
[   36.566303] CPU: 6 PID: 533 Comm: v4l2-ctl Not tainted 4.8.0
[   36.574466] Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
[   36.580526] task: ee3cc600 task.stack: ed626000
[   36.585046] PC is at try_module_get+0x1c/0xac
[   36.589364] LR is at try_module_get+0x1c/0xac
[   36.593698] pc : [<c0187a60>]    lr : [<c0187a60>]    psr: 80070013
[   36.593698] sp : ed627de0  ip : a0070013  fp : 00000000
[   36.605156] r10: 00000002  r9 : ed627ed0  r8 : 00000000
[   36.610331] r7 : c01e5f14  r6 : ed57be00  r5 : bf1f8200  r4 : bf1f8200
[   36.616834] r3 : 00000002  r2 : 00000002  r1 : 01930192  r0 : 00000001
..
[   36.785004] [<c0187a60>] (try_module_get) from [<c01e5c10>] (cdev_get+0x1c/0x4c)
[   36.792362] [<c01e5c10>] (cdev_get) from [<c01e5f40>] (chrdev_open+0x2c/0x178)
[   36.799555] [<c01e5f40>] (chrdev_open) from [<c01df5d4>] (do_dentry_open+0x1e0/0x300)
[   36.807360] [<c01df5d4>] (do_dentry_open) from [<c01eecdc>] (path_openat+0x35c/0xf58)
[   36.815154] [<c01eecdc>] (path_openat) from [<c01f0668>] (do_filp_open+0x5c/0xc0)
[   36.822606] [<c01f0668>] (do_filp_open) from [<c01e09ac>] (do_sys_open+0x10c/0x1bc)
[   36.830235] [<c01e09ac>] (do_sys_open) from [<c01078c0>] (ret_fast_syscall+0x0/0x3c)
[   36.837942] Code: 0a00001c e1a04000 e3a00001 ebfec92d (e5943000)

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>