libertas: improve function init/shutdown handling for SD8688

This patch is to incorporate Dan Williams' comments for commit:
"libertas: implement function init/shutdown commands for SD8688"

1. remove fn_init_required and fn_shutdown_required variables from
lbs_private structure. If required, __lbs_cmd() will be called
directly to send function init/shutdown command for SD8688 in
if_sdio_probe() or if_sdio_remove() callback.

2. add global variable "user_rmmod" to distinguish between the module
removal case and the card removal case. This flag will be checked in
if_sdio_remove() against SD8688 card to determine whether or not the
function shutdown command needs to be sent.

3. remove "card" from if_sdio_model structure as it cannot store
card pointers for multiple cards. Besides, it's no longer needed
to store the "card" pointer with changes #1 & #2 above.

Signed-off-by: Bing Zhao <bzhao@marvell.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>