staging: rtl8712: change SupportedRates to rates

Change the value to a name that conforms to Linux coding style.

"rates" is equally expressive in this context, and I have left alone
a comment and function name that describe the rates as supported rates.

Signed-off-by: Joshua Clayton <stillcompiling@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>