staging: rtl8712: remove wrapper function _init_listhead

_init_listhead is just an inline wrapper around INIT_LIST_HEAD.  This
patch removes the wrapper and directly uses INIT_LIST_HEAD instead.

Signed-off-by: James A Shackleford <shack@linux.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>