Move the I2C speed config key out of the choice prompt.

Kconfig

@@ -12,13 +12,14 @@ menu "BMX280 Options"
             bool "I2C Master Driver (i2c_master.h)"
             help
                 Use I2C Master Driver (i2c_master.h) for ESP-IDF >= 5.3.
+    endchoice
+    
     config BMX280_I2C_CLK_SPEED_HZ
         int "I2C Clock Speed (Hz)"
         default 100000
         range 1000 400000
         help
-                Set the I2C clock speed in Hz.
-    endchoice
+            Set the I2C clock speed in Hz. Only applies to the new I2C Master driver.
     
     choice BMX280_EXPECT_DEVICE
         prompt "Installed Sensor Model"