s2io: Fixes in updating skb->truesize and code cleanup.

1. Fix for updating skb->truesize properly.
2. Disable NAPI only if more than one ring configured in case of MSI/MSI-X
   interrupts. Previously we were disabling NAPI irrespective of number of
   rings when MSI/MSI-X interrupts were used.
3. Code cleanup.

Signed-off-by: Sivakumar Subramani <sivakumar.subramani@neterion.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>