mwifiex: add rx histogram statistics support

This patch add a new debugfs item histogram used for reporting
rx data packet statitics(rx rate, snr, noise floor, signal strenth)
to userspace.

Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>