bna: Minor IRQ Index and Definition Change

Change details:
 - Always set mbox irq index to zero and make corresponding changes in mbox
   irq alloc/sync function and txrx irq index calculation. Add definition of
   BNAD_INTX_TX_IB_BITMASK & BNAD_INTX_RX_IB_BITMASK and update
   bnad_txrx_irq_alloc accordingly.

Signed-off-by: Rasesh Mody <rmody@brocade.com>
Signed-off-by: David S. Miller <davem@davemloft.net>