usb: gadget: f_midi: Fix memleak in f_midi_alloc

usb: gadget: f_midi: Fix memleak in f_midi_alloc

In the error path, if midi is not null, we should
free the midi->id if necessary to prevent memleak.

Fixes: b85e9de9e818d ("usb: gadget: f_midi: convert to new function interface with backward compatibility")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhang Qilong <zhangqilong3@huawei.com>
Link: https://lore.kernel.org/r/20201117021629.1470544-2-zhangqilong3@huawei.com
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>