usb: gadget: f_hid: fixed NULL pointer dereference

Disconnecting and reconnecting the USB cable can lead to crashes
and a variety of kernel log spam.

The problem was found and reproduced on the Raspberry Pi [1]
and the original fix was created in Raspberry's own fork [2].

Link: https://github.com/raspberrypi/linux/issues/3870 [1]
Link: https://github.com/raspberrypi/linux/commit/a6e47d5f4efbd2ea6a0b6565cd2f9b7bb217ded5

 [2]
Signed-off-by: Maxim Devaev <mdevaev@gmail.com>
Signed-off-by: Phil Elwell <phil@raspberrypi.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210723155928.210019-1-mdevaev@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>