xtensa: remove @return doc for void functions

For functions returning nothing, there is no need to document
with @return, as Doxgen complains about "documented empty
return type of ...".

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

arch/xtensa/core/irq_manage.c

@@ -7,7 +7,8 @@
 #include <stdio.h>
 #include <arch/xtensa/irq.h>
 #include <sys/__assert.h>
-/*
+
+/**
  * @internal
  *
  * @brief Set an interrupt's priority
@@ -22,10 +23,7 @@
  * Valid values are from 1 to 6. Interrupts of priority 1 are not masked when
  * interrupts are locked system-wide, so care must be taken when using them.
  * ISR installed with priority 0 interrupts cannot make kernel calls.
- *
- * @return N/A
  */
-
 void z_irq_priority_set(unsigned int irq, unsigned int prio, uint32_t flags)
 {
 	__ASSERT(prio < XCHAL_EXCM_LEVEL + 1,