kinetis:adc fix typo vailid->valid (#8054)

Fix typo on master

src/drivers/kinetis/adc/adc.cpp

@@ -392,10 +392,10 @@ ADC::update_system_power(hrt_abstime now)
 	system_power.usb_connected = BOARD_ADC_USB_CONNECTED;
 	/* If provided used the Valid signal from HW*/
 #if defined(BOARD_ADC_USB_VALID)
-	system_power.usb_vaild = BOARD_ADC_USB_VALID;
+	system_power.usb_valid = BOARD_ADC_USB_VALID;
 #else
 	/* If not provided then use connected */
-	system_power.usb_vaild  = system_power.usb_connected;
+	system_power.usb_valid  = system_power.usb_connected;
 #endif
 
 	system_power.brick_valid   = BOARD_ADC_BRICK_VALID;