platform/x86: intel_ips: fix set but unused warning in read_mgtv

drivers/platform/x86/intel_ips.c:832:6: warning: variable ‘ret’ set but
not used [-Wunused-but-set-variable]
832 | u16 ret;
| ^~~

Fix it by mark ret as '__maybe_unused'.

Reported-by: Hulk Robot <[e-mail]>
Signed-off-by: yangerkun <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Hans de Goede <[e-mail]>

drivers/platform/x86/intel_ips.c

···
 
 static u16 read_mgtv(struct ips_driver *ips)
 {
-	u16 ret;
+	u16 __maybe_unused ret;
 	u64 slope, offset;
 	u64 val;