usb: gadget: f_diag: allocate diag USB channel when not found

Currently usb function instance driver depends on diag char driver to
create the channel (using usb_diag_open api). Failing to create channel
can result in enumeration failure. Avoid this dependency by creating
diag channel from function instance if not available. Same channel
will be reused when diag char driver creates/opens the channel.

Change-Id: Ie9f1b5d87257eb019552a3121abc77b7fb8ac544
Signed-off-by: Liangliang Lu <luliang@codeaurora.org>