spidernet: fix misnamed flag

The transmit frame tail bit is stranglely misnamed as
"no checksum". Fix the name to what it should be:
"transmit frame tail". No functional change,
just a name change.

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