mwifiex: remove wireless.h inclusion and fix resulting bugs

replace IW_MAX_AP & IW_CUSTOM_MAX with local definitions
and remove usage of struct iw_statistics.

Cc: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>