mwifiex: correct histogram data with appropriate index

BugLink: http://bugs.launchpad.net/bugs/1815234
[ Upstream commit 30bfce0b63fa68c14ae1613eb9d259fa18644074 ]

Correct snr/nr/rssi data index to avoid possible buffer underflow.

Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>