USB: cdc-wdm: no need to fill the in request URB every time it's submitted

Filling the same URB with the exact same data is pointless.  It can be filled
once and resubmitted.  And not doing buffer allocation and URB filling at the
same place only serves to hide size mismatch bugs

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>