[PATCH] USBATM: handle urbs containing partial cells

The receive logic has always assumed that urbs contain an integral
number of ATM cells, which is a bit naughty, though it never caused
any problems with bulk transfers.  Isochronous urbs spank us soundly
for this.  Fixed thanks to this patch, mostly by Stanislaw Gruszka.

Signed-off-by: Duncan Sands <baldrick@free.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>