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 <hulkci@huawei.com>
Signed-off-by: yangerkun <yangerkun@huawei.com>
Link: https://lore.kernel.org/r/20210607014702.2981097-1-yangerkun@huawei.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>