bnx2x: Add missing afex code

Commit a3348722 added afex support but lacked
several logical changes. This lack can cause afex to crash, and also
have a slight effect on other flows (i.e., driver always assumes the Tx ring
has less available buffers than what it actually has).

This patch adds the missing segments, fixing said issues.

Signed-off-by: Yuval Mintz <yuvalmin@broadcom.com>
Signed-off-by: Barak Witkowski <barak@broadcom.com>
Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>