sky2: turn of dynamic Tx watermark workaround (FE+ only)

Add workaround for issues FE+ (A0) transmit watermark.
This is copied verbatim from vendor driver sk98lin (10.22.4.3).
Don't have that chip version and no more information seems to be available.

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