wifi: mwifiex: cleanup struct mwifiex_sdio_mpa_rx

Drop filled with NULL pointers but otherwise unused 'skb_arr'
array of 'struct mwifiex_sdio_mpa_rx', adjust related code.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Acked-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230821115254.167552-1-dmantipov@yandex.ru