usb: f_cdev: USB remote wake up feature implementation for DUN

Implement usb remote wake up feature for dun on sdx12.
For super speed this feature supports wake up of a
particular function from suspend, where as for high speed,
remote wakeup doesn't support wakeup of a particular function
from suspend, but entire usb bus needs to wakeup. When usb_func_wakeup
in composite driver fails because it needs to wake usb bus before
function wakeup is triggered, -EAGAIN is returned which cannot be
considered as a function wakeup failure. Hence add proper handling
in function driver for EAGAIN return case.

Change-Id: I15c18a4a7203e82c74ad1b4905c08eff91d91b4f
Signed-off-by: Rohith Kollalsi <rkollals@codeaurora.org>