mwifiex: get rid of global adapter pointer

1) Avoid global adapter pointer usage
   a) in sdio.c by moving some code from mwifiex_cleanup_module() to
      mwifiex_sdio_remove().
   b) in main.c by passing an extra parameter to few functions.
2) Add new variable "user_rmmod" to identify if mwifiex_sdio_remove()
   callback function is called in card removal or rmmod context. These
   code changes are adapted from Libertas driver.
3) Remove unnecessary NULL pointer check for "func" pointer in
   mwifiex_sdio_remove().

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>