USB: cdc-wdm: cleanup error codes

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The internal error codes returned in the write() code
path cannot be simply passed on to user space.

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