cc2538: lpm: Fix build warnings with LPM_CONF_ENABLE == 0

Some arguments passed to the functions disabled with
LPM_CONF_ENABLE == 0 could trigger build warnings because they became
unused with this configuration option. Disable these functions by using
empty static inline functions instead of empty macros, so that the
function arguments are always considered by the compiler as used,
without having to #if-out code in many places.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau.dev@gmail.com>

cpu/cc2538/lpm.h

@@ -79,6 +79,9 @@ extern rtimer_clock_t lpm_stats[3];
 #define LPM_PM2           2
 /** @} */
 /*---------------------------------------------------------------------------*/
+typedef bool (*lpm_periph_permit_pm1_func_t)(void);
+
+#if LPM_CONF_ENABLE
 /**
  * \brief Initialise the LPM module
  */
@@ -185,8 +188,6 @@ void lpm_exit(void);
  * \sa lpm_enter()
  */
 void lpm_set_max_pm(uint8_t pm);
-/*---------------------------------------------------------------------------*/
-typedef bool (*lpm_periph_permit_pm1_func_t)(void);
 
 /**
  * \brief Register a peripheral function which will get called by the LPM
@@ -207,12 +208,18 @@ typedef bool (*lpm_periph_permit_pm1_func_t)(void);
 void lpm_register_peripheral(lpm_periph_permit_pm1_func_t permit_pm1_func);
 /*---------------------------------------------------------------------------*/
 /* Disable the entire module if required */
-#if LPM_CONF_ENABLE==0
+#else
 #define lpm_init()
 #define lpm_enter()
 #define lpm_exit()
-#define lpm_set_max_pm(...)
-#define lpm_register_peripheral(...)
+static inline void
+lpm_set_max_pm(uint8_t pm)
+{
+}
+static inline void
+lpm_register_peripheral(lpm_periph_permit_pm1_func_t permit_pm1_func)
+{
+}
 #endif
 
 #endif /* LPM_H_ */