staging: r8188eu: replace mlme_handler with function pointer

mlme_sta_tbl is an array of struct mlme_handler. mlme_handler's num and
str components are not used. The code in mgt_dispatcher uses the subtype
number of the incoming frame to select the array entry for compiling the
repsonse.

We can remove struct mlme_handler and make mlme_sta_tbl an array of
function pointers.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220417102221.466524-5-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>