bnxt_en: Put the TX producer information in the TX BD opaque field

Currently, the opaque field in the TX BD is only used for debugging.
The TX completion logic relies on getting one TX completion for each
packet and they always complete in order.

Improve this scheme by putting the producer information (ring index plus
number of BDs for the packet) in the opaque field.  This way, we can
handle TX completion processing by looking at the last TX completion
instead of counting the number of completions.

Since we no longer need to count the exact number of completions, we can
optimize xmit_more by disabling TX completion when the xmit_more
condition is true.  This will be done in later patches.

This patch is only initializing the opaque field in the TX BD and is
not changing the driver's TX completion logic yet.

Reviewed-by: Andy Gospodarek <gospo@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>