parisc: Fix compiler warnings in debug_core.c

Fix this compiler warning:
kernel/debug/debug_core.c: In function ‘kgdb_cpu_enter’:
arch/parisc/include/asm/cmpxchg.h:48:3: warning: value computed is not used [-Wunused-value]
   48 |  ((__typeof__(*(ptr)))__xchg((unsigned long)(x), (ptr), sizeof(*(ptr))))
arch/parisc/include/asm/atomic.h:78:30: note: in expansion of macro ‘xchg’
   78 | #define atomic_xchg(v, new) (xchg(&((v)->counter), new))
      |                              ^~~~
kernel/debug/debug_core.c:596:4: note: in expansion of macro ‘atomic_xchg’
  596 |    atomic_xchg(&kgdb_active, cpu);
      |    ^~~~~~~~~~~

Signed-off-by: Helge Deller <deller@gmx.de>