ipx: restore token ring define to include/linux/ipx.h

Commit 211ed865

    "net: delete all instances of special processing for token ring"

removed the define for IPX_FRAME_TR_8022.

While it is unlikely, we can't be 100% sure that there aren't
random userspace consumers of this value, so restore it.

The only instance I could find was in ncpfs-2.2.6, and it was
safe as-is, since it used #ifdef IPX_FRAME_TR_8022 around the
two use cases it had, but there may be other userspace packages
without similar ifdefs.

Cc: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>