adapter: make adapter_config_khz() static

The function is not referenced outside the file.
Make it static.

Change-Id: I72e96624749ae4cc7f4566d737a88186e899616a
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8541
Tested-by: jenkins

src/jtag/adapter.h

@@ -97,9 +97,6 @@ int adapter_get_speed(int *speed);
  */
 int adapter_get_speed_readable(int *speed);
 
-/** Attempt to configure the adapter for the specified kHz. */
-int adapter_config_khz(unsigned int khz);
-
 /**
  * Attempt to enable RTCK/RCLK. If that fails, fallback to the
  * specified frequency.