platform/x86: intel_pmc_core: Fix "unsigned 'ret' is never less than zero" smatch warning

Change the type of ret form a size_t to a ssize_t, matching the prototype
of simple_write_to_buffer(), fixing this warning reported by smatch:

drivers/platform/x86/intel_pmc_core.c:1369 pmc_core_lpm_latch_mode_write() warn: unsigned 'ret' is never less than zero.

Fixes: 8074a79fad ("platform/x86: intel_pmc_core: Add option to set/clear LPM mode")
Cc: David E. Box <david.e.box@linux.intel.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20210419143109.30612-1-hdegoede@redhat.com

drivers/platform/x86/intel_pmc_core.c

@@ -1360,7 +1360,7 @@ static ssize_t pmc_core_lpm_latch_mode_write(struct file *file,
 	struct pmc_dev *pmcdev = s->private;
 	bool clear = false, c10 = false;
 	unsigned char buf[8];
-	size_t ret;
+	ssize_t ret;
 	int idx, m, mode;
 	u32 reg;