mwifiex: add hscfg to debugfs

Some SDIO controllers do not support MMC_PM_KEEP_POWER properly.
To test host sleep feature without putting the system into sleep
we need to simulate host sleep configuration & handshake between
driver and firmware using customized parameters.
This patch adds hscfg debugfs item, with which user could change
host sleep parameters for debugging.

Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>