gdm72xx: remove unnecessary cast in gdm_wimax.c

This patch removes an unnecessary cast on the return value
of alloc_netdev(), since alloc_netdev() returns a pointer
to the allocated struct net_device anyway.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>