bnx2x: Fix potential incorrect link speed provision

Fix possible incorrect link speed provision following rapid link speed change.
Clear link speed mask after each link change, and not only after link down.

Signed-off-by: Yaniv Rosner <yanivr@broadcom.com>
Signed-off-by: Barak Witkowski <barak@broadcom.com>
Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>