net: phy: bcm7xxx: update workaround to fix 100BaseT corner cases

Update the AFE_TX_CONFIG value to solve marginal rise/fall issues
observed when the link is operating in 100BaseT. This workaround applies
to GPHY revisions D0, E0 and newer.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>