drivers/pcf857x: Move compile time check to compilation unit

This allows including the header without using the module. Obviously,
calls to the functions provided by the header won't like without using
the module. But including the header can still be useful for e.g.:

    if (IS_USED(MODULE_PCF857x)) {
        /* make use of the module */
    }

In the above example all calls to pcf857x functions would be optimized
out when the module is not used, full compile checks happen in either
case.

drivers/include/pcf857x.h

@@ -260,10 +260,6 @@ extern "C"
 #include "event.h"
 #endif /* MODULE_PCF857X_IRQ */
 
-#if !IS_USED(MODULE_PCF8574) && !IS_USED(MODULE_PCF8574A) && !IS_USED(MODULE_PCF8575)
-#error "Please provide a list of pcf857x variants used by the application (pcf8574, pcf8574a or pcf8575)"
-#endif
-
 /**
  * @name    PCF857X I2C slave addresses
  *

drivers/pcf857x/pcf857x.c

@@ -42,6 +42,10 @@
 
 #endif /* ENABLE_DEBUG */
 
+#if !IS_USED(MODULE_PCF8574) && !IS_USED(MODULE_PCF8574A) && !IS_USED(MODULE_PCF8575)
+#error "Please provide a list of pcf857x variants used by the application (pcf8574, pcf8574a or pcf8575)"
+#endif
+
 #if IS_USED(MODULE_PCF857X_IRQ_LOW)
 #define PCF857X_EVENT_PRIO EVENT_PRIO_LOWEST
 #elif IS_USED(MODULE_PCF857X_IRQ_MEDIUM)