usb: f_ccid: Assign descriptors only in function bind

Currently we assign the FS, HS and SS descriptors in bind_config
unconditionally. If the max supported speed is high, then bind
function does not allocate SS descriptors but the unbind function
calls usb_free_all_descriptors which results in slab error since
the SS descriptors exist anyway.
Fix this by assigning any function descriptors during bind only.

Change-Id: I83384bd68563e6b94fbb7c1f437282a3fa524de7
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>