mailbox: PCC: handle parse error

acpi_pcc_probe() calls acpi_table_parse_entries_array() but fails
to check for an error return.  This in turn can result in calling
kcalloc() with a negative count as well as emitting the following
misleading erorr message:

[    2.642015] Could not allocate space for PCC mbox channels

Fixes: 8f8027c5f935 (mailbox: PCC: erroneous error message when parsing ACPI PCCT)
Signed-off-by: David Arcari <darcari@redhat.com>
Reviewed-by: Al Stone <ahs3@redhat.com>
Cc: 4.18+ <stable@vger.kernel.org> # 4.18+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/mailbox/pcc.c

@@ -461,8 +461,11 @@ static int __init acpi_pcc_probe(void)
 	count = acpi_table_parse_entries_array(ACPI_SIG_PCCT,
 			sizeof(struct acpi_table_pcct), proc,
 			ACPI_PCCT_TYPE_RESERVED, MAX_PCC_SUBSPACES);
-	if (count == 0 || count > MAX_PCC_SUBSPACES) {
-		pr_warn("Invalid PCCT: %d PCC subspaces\n", count);
+	if (count <= 0 || count > MAX_PCC_SUBSPACES) {
+		if (count < 0)
+			pr_warn("Error parsing PCC subspaces from PCCT\n");
+		else
+			pr_warn("Invalid PCCT: %d PCC subspaces\n", count);
 		return -EINVAL;
 	}