usb: gadget: storage: Remove warning message

This change is to fix below warning message in following scenario:
usb_composite_setup_continue: Unexpected call

When system tried to enter suspend, the fsg_disable() will be called to
disable fsg driver and send a signal to fsg_main_thread. However, at
this point, the fsg_main_thread has already been frozen and can not
respond to this signal. So, this signal will be pended until
fsg_main_thread wakes up.

Once system resumes from suspend, fsg_main_thread will detect a signal
pended and do some corresponding action (in handle_exception()). Then,
host will send some setup requests (get descriptor, set configuration...)
to UDC driver trying to enumerate this device. During the handling of "set
configuration" request, it will try to sync up with fsg_main_thread by
sending a signal (which is the same as the signal sent by fsg_disable)
to it. In a similar manner, once the fsg_main_thread receives this
signal, it will call handle_exception() to handle the request.

However, if the fsg_main_thread wakes up from suspend a little late and
"set configuration" request from Host arrives a little earlier,
fsg_main_thread might come across the request from "set configuration"
when it handles the signal from fsg_disable(). In this case, it will
handle this request as well. So, when fsg_main_thread tries to handle
the signal sent from "set configuration" later, there will nothing left
to do and warning message "Unexpected call" is printed.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: EJ Hsu <ejh@nvidia.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>