[rt2x00-users] Rework USB padding handling in rt2800usb

[Jakub Kiciński]

Hi!

As Ivo suggested USB padding should be handled by common code so I
modified rt2800usb not to add the padding on its own and slightly
changed rt2x00usb code to ensure padding is always zeroed.

My previous posting of this patch was a little messy, sorry for that.
I post it again divided in two "logical changes" and with better
descriptions. Hopefully this time it will raise some comments.

  -- Kuba