usb: gadget: f_cdev: Fix memory leak upon composition switch

Driver allocates notify request in function bind() callback
but frees it in function disable() callback. Upon composition
switch adb daemon gets restarted causing function bind/unbind
to happen twice. This results into allocating notify request
memory twice and only freeing it once either upon disconnect
or next composition switch. Fix the issue freeing memory in
function unbind callback.

Change-Id: I7716cff99b824b55e48aea57b9da91c16a8ecac1
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>