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: I11debd0189d81542762af22b3d203728d2266a42
Signed-off-by: Liangliang Lu <luliang@codeaurora.org>