spidernet: turn off descriptor chain end interrupt.

At some point, the transmit descriptor chain end interrupt (TXDCEINT)
was turned on. This is a mistake; and it damages small packet
transmit performance, as it results in a huge storm of interrupts.
Turn it off.

Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>