pegasus: fixes URB buffer allocation size;

usb_fill_bulk_urb() receives buffer length parameter 8 bytes larger
than what's allocated by alloc_skb(); This seems to be a problem with
older (pegasus usb-1.1) devices, which may silently return more data
than the maximal packet length.

Reported-by: Lincoln Ramsay <a1291762@gmail.com>
Signed-off-by: Petko Manolov <petkan@mip-labs.com>
Signed-off-by: David S. Miller <davem@davemloft.net>