staging:rtl8192u: Make HTMcsToDataRate static - Style

The function HTMcsToDataRate() is not used outside the file in which
it is defined, so has been declared as 'static' and the prototype
removed from the header file.

This is a coding style change which should have no impact on runtime
code execution.

Signed-off-by: John Whitmore <johnfwhitmore@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>