staging: vt6656: vnt_rf_setpower: fix missing rate RATE_12M

When the driver sets this rate a power of zero value is set causing
data flow stoppage until another rate is tried.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Cc: <stable@vger.kernel.org> # v3.17+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>