net/cadence_gem: Fix rx multi-fragment packets

net/cadence_gem: Fix rx multi-fragment packets

Bytes_to_copy was being updated before its final use where it
advances the rx buffer pointer. This was causing total mayhem,
where packet data for any subsequent fragments was being fetched
from the wrong place.

Reported-by: Deepika Dhamija <[email protected]>
Signed-off-by: Peter Crosthwaite <[email protected]>
Reviewed-by: Peter Maydell <[email protected]>
Message-id: c2a1c65c1fd06eb274442a0fa4a6839d940e145e.1386136219[email protected]
Signed-off-by: Peter Maydell <[email protected]>