mwifiex: get rid of global save_adapter and sdio_work

This patch moves sdio_work to card structure, in this way we can get
adapter structure in the work, so save_adapter won't be needed.

Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>