tg3: Relax 5717 serdes restriction

tg3 is coded to refuse to attach to 5717 serdes devices.  Now that the
hardware is better supported, we can relax this restriction.  This patch
also fixes a recently introduced bug which will cause serdes parallel
detection not to work with 5780 class devices.

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