enic: Get/Set interrupt resource index for transmit and receive queues

Instead of deriving the index of a transmit/receive interrupt resource
from the transmit/receive queue index, always save and retrieve it
using an additional variable.

Signed-off-by: Christian Benvenuti <benve@cisco.com>
Signed-off-by: Danny Guo <dannguo@cisco.com>
Signed-off-by: Vasanthy Kolluri <vkolluri@cisco.com>
Signed-off-by: Roopa Prabhu <roprabhu@cisco.com>
Signed-off-by: David Wang <dwang2@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>