ANDROID: usb: f_accessory: Don't drop NULL reference in acc_disconnect()

If get_acc_dev() fails to obtain a reference to the current device,
acc_disconnect() will attempt to put_acc_dev() with the resulting NULL
pointer, leading to a crash:

 | Unable to handle kernel NULL pointer dereference at virtual address 00000074
 | [...]
 | [<c0abb288>] (acc_disconnect) from [<c0a91a38>] (android_disconnect+0x1c/0x7c)
 | [<c0a91a38>] (android_disconnect) from [<c0a93958>] (usb_gadget_udc_reset+0x10/0x34)
 | [<c0a93958>] (usb_gadget_udc_reset) from [<c0a4a9c4>] (dwc3_gadget_reset_interrupt+0x88/0x4fc)
 | [<c0a4a9c4>] (dwc3_gadget_reset_interrupt) from [<c0a491f8>] (dwc3_process_event_buf+0x60/0x3e4)
 | [<c0a491f8>] (dwc3_process_event_buf) from [<c0a49180>] (dwc3_thread_interrupt+0x24/0x3c)
 | [<c0a49180>] (dwc3_thread_interrupt) from [<c02b3404>] (irq_thread_fn+0x1c/0x58)
 | [<c02b3404>] (irq_thread_fn) from [<c02b326c>] (irq_thread+0x1ec/0x2f4)
 | [<c02b326c>] (irq_thread) from [<c0260804>] (kthread+0x1a8/0x1ac)
 | [<c0260804>] (kthread) from [<c0200138>] (ret_from_fork+0x14/0x3c)

Follow the pattern used elsewhere, and return early if we fail to obtain
a reference.

Bug: 173789633
Reported-by: YongQin Liu <yongqin.liu@linaro.org>
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I37a2bff5bc1b6b8269788d08191181763bf0e896
Signed-off-by: Giuliano Procida <gprocida@google.com>