staging: rtl8192e: rtllib_HTProc: Make functions static

Make as many functions as possible static in rtllib_HTProc.c.

The following functions were affected:
- HTMcsToDataRate
- HTFilterMCSRate

Signed-off-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>