atl1: change cmb write threshold

Change the threshold number of descriptors used to trigger CMB writes.
The vendor reports that under certain conditions this will reduce the
number of unnecessary tx interrupts and improve rx performance.

This change is lifted directly from vendor version 1.2.40.2 of the L1
driver.

Signed-off-by: Jay Cliburn <jacliburn@bellsouth.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>