tg3: Reduce indent level of tg3_rx_prodring_alloc

This patch adds an inverted "jumbo ring enable" test and jumps to the exit
if it succeeds.  The change reduces the indent level of the remaining
code making it more readable.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>