usb: cdc-wdm: make reset work with blocking IO

Add a flag to tell wdm_read/wdm_write that a reset is in progress,
and wake any blocking read/write before taking the mutexes.  This
allows the device to reset without waiting for blocking IO to
finish.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Acked-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>