mwifiex: export threshold value and retry limit to cfg80211

While registering the driver with cfg80211, update the threshold
value and retry limit to cfg80211.

Signed-off-by: Ujjal Roy <royujjal@gmail.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>