ibmvnic: Use/rename local vars in init_tx_pools

Use/rename local variables in init_tx_pools() for consistency with
init_rx_pools() and for readability. Also add some comments

Reviewed-by: Rick Lindsley <ricklind@linux.vnet.ibm.com>
Reviewed-by: Dany Madden <drt@linux.ibm.com>
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>