drivers/net/pppol2tp.c: remove exceptional & on function name

drivers/net/pppol2tp.c: remove exceptional & on function name

In this file, function names are otherwise used as pointers without &.

A simplified version of the semantic patch that makes this change is as
follows: (http://coccinelle.lip6.fr/)

// <smpl>
@r@
identifier f;
@@

f(...) { ... }

@@
identifier r.f;
@@

- &f
+ f
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>