bnx2x: Add new SW stat 'tx_exhaustion_events'

Driver already has an internal counter for number of times a given queue
had to be stopped due to Tx ring exhaustion.
This add the counter to the statistics presented by driver, e.g., by using
`ethtool -S'.

Signed-off-by: Yuval Mintz <Yuval.Mintz@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>