staging: rtl8712: remove wrapper function list_insert_tail

list_insert_tail is just an inline wrapper around list_add_tail.  This
patch removes the wrapper and directly uses list_add_tail.

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