Spidernet Avoid possible RX chain corruption

Delete possible source of chain corruption; the hardware
already knows the location of the tail, and writing it
again is likely to mess it up.

Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Cc: James K Lewis <jklewis@us.ibm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jeff Garzik <jeff@garzik.org>