sky2: be more selective about FIFO watchdog

Be more selective about when to enable the ram buffer watchdog code.
It is unnecessary on XL A3 or later revs, and with Yukon FE
the buffer is so small (4K) that the watchdog detects false positives.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>