x86: apm: avoid uninitialized data

apm_bios_call() can fail, and return a status in its argument structure.
If that status however is zero during a call from
apm_get_power_status(), we end up using data that may have never been
set, as reported by "gcc -Wmaybe-uninitialized":

  arch/x86/kernel/apm_32.c: In function ‘apm’:
  arch/x86/kernel/apm_32.c:1729:17: error: ‘bx’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  arch/x86/kernel/apm_32.c:1835:5: error: ‘cx’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  arch/x86/kernel/apm_32.c:1730:17: note: ‘cx’ was declared here
  arch/x86/kernel/apm_32.c:1842:27: error: ‘dx’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  arch/x86/kernel/apm_32.c:1731:17: note: ‘dx’ was declared here

This changes the function to return "APM_NO_ERROR" here, which makes the
code more robust to broken BIOS versions, and avoids the warning.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Jiri Kosina <jkosina@suse.cz>
Reviewed-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/kernel/apm_32.c

@@ -1042,8 +1042,11 @@ static int apm_get_power_status(u_short *status, u_short *bat, u_short *life)
 
 	if (apm_info.get_power_status_broken)
 		return APM_32_UNSUPPORTED;
-	if (apm_bios_call(&call))
+	if (apm_bios_call(&call)) {
+		if (!call.err)
+			return APM_NO_ERROR;
 		return call.err;
+	}
 	*status = call.ebx;
 	*bat = call.ecx;
 	if (apm_info.get_power_status_swabinminutes) {