tg3: Limit CLKREQ fix to A[01] of 57780 asic rev

This patch restricts the CLKREQ bugfix to the A0 and A1 revisions
of 57780 ASIC rev chips.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>