wimax: fix oops in wimax_dev_get_by_genl_info() when looking up non-wimax iface

When a non-wimax interface is looked up by the stack, a bad pointer is
returned when the looked-up interface is not found in the list (of
registered WiMAX interfaces). This causes an oops in the caller when
trying to use the pointer.

Fix by properly setting the pointer to NULL if we don't exit from the
list_for_each() with a found entry.

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>