mwifiex: make function mwifiex_ret_pkt_aggr_ctrl static

function mwifiex_ret_pkt_aggr_ctrl can be made static as it does not
need to be in global scope.

Cleans up sparse warning: "symbol 'mwifiex_ret_pkt_aggr_ctrl' was not
declared. Should it be static?"

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>