powerpc/process: Fix uninitialised variable error

Clang, and GCC with -Wmaybe-uninitialized, can't see that val is
unused in get_fpexec_mode():

  arch/powerpc/kernel/process.c:1940:7: error: variable 'val' is used
  uninitialized whenever 'if' condition is true
		  if (cpu_has_feature(CPU_FTR_SPE)) {
		      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

We know that CPU_FTR_SPE will only be true iff CONFIG_SPE is also
true, but the compiler doesn't.

Avoid it by initialising val to zero.
Reported-by: kernel test robot <lkp@intel.com>
Fixes: 532ed190 ("powerpc/process: Remove useless #ifdef CONFIG_SPE")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20200917024509.3253837-1-mpe@ellerman.id.au

arch/powerpc/kernel/process.c

@@ -1934,7 +1934,7 @@ int set_fpexc_mode(struct task_struct *tsk, unsigned int val)
 
 int get_fpexc_mode(struct task_struct *tsk, unsigned long adr)
 {
-	unsigned int val;
+	unsigned int val = 0;
 
 	if (tsk->thread.fpexc_mode & PR_FP_EXC_SW_ENABLE) {
 		if (cpu_has_feature(CPU_FTR_SPE)) {