ANDROID: usb: gadget: f_midi: set fi->f to NULL when free f_midi function

fi->f is set in f_midi's alloc_func, need to clean this to
NULL in free_func, otherwise on ConfigFS's function switch,
midi->usb_function it self is freed, fi->f will be a wild
pointer and run into below kernel panic:
---------------
[   58.950628] Unable to handle kernel paging request at virtual address 63697664
[   58.957869] pgd = c0004000
[   58.960583] [63697664] *pgd=00000000
[   58.964185] Internal error: Oops: 80000005 [xin3liang#1] PREEMPT SMP ARM
[   58.970111] Modules linked in:
[   58.973191] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.1.15-03504-g34c857c-dirty torvalds#89
[   58.981024] Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree)
[   58.987557] task: c110bd70 ti: c1100000 task.ti: c1100000
[   58.992962] PC is at 0x63697664
[   58.996120] LR is at android_setup+0x78/0x138
<..snip..>
[   60.044980] 1fc0: ffffffff ffffffff c1000684 00000000 00000000 c108ecd0 c11f7294 c11039c0
[   60.053181] 1fe0: c108eccc c110d148 1000406 412fc09a 00000000 1000807c 00000000 00000000
[   60.061420] [<c073b1fc>] (android_setup) from [<c0730490>] (udc_irq+0x758/0x1034)
[   60.068951] [<c0730490>] (udc_irq) from [<c017c650>] (handle_irq_event_percpu+0x50/0x254)
[   60.077165] [<c017c650>] (handle_irq_event_percpu) from [<c017c890>] (handle_irq_event+0x3c/0x5c)
[   60.086072] [<c017c890>] (handle_irq_event) from [<c017f3ec>] (handle_fasteoi_irq+0xe0/0x198)
[   60.094630] [<c017f3ec>] (handle_fasteoi_irq) from [<c017bcfc>] (generic_handle_irq+0x2c/0x3c)
[   60.103271] [<c017bcfc>] (generic_handle_irq) from [<c017bfb8>] (__handle_domain_irq+0x7c/0xec)
[   60.112000] [<c017bfb8>] (__handle_domain_irq) from [<c0101450>] (gic_handle_irq+0x24/0x5c)
--------------

Signed-off-by: Winter Wang <[email protected]>

Author: Winter Wang

drivers/usb/gadget/function/f_midi.c

@@ -1140,6 +1140,7 @@ static void f_midi_free(struct usb_function *f)
 	for (i = opts->in_ports - 1; i >= 0; --i)
 		kfree(midi->in_port[i]);
 	kfree(midi);
+	opts->func_inst.f = NULL;
 	--opts->refcnt;
 	mutex_unlock(&opts->lock);
 }