add NAPI support to sb1250-mac.c

Patch to add NAPI support to sb1250-mac.c (rev 2).  This patch differs from
the last in that the NAPI support isn't marked as experimental, nor is it
configurable (ie.  once applied - NAPI is enabled all the time).  This was
based on feedback from Ralf and others.
Signed-off-by: Mark Mason <mason@broadcom.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>