RISC-V: ACPI: Update the return value of acpi_get_rhct()

acpi_get_rhct() currently returns pointer to acpi_table_header
structure. But since this is specific to RHCT, return pointer to
acpi_table_rhct structure itself.

Suggested-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20231018124007.1306159-3-sunilvl@ventanamicro.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: vlsunil

drivers/acpi/riscv/rhct.c

@@ -9,7 +9,7 @@
 
 #include <linux/acpi.h>
 
-static struct acpi_table_header *acpi_get_rhct(void)
+static struct acpi_table_rhct *acpi_get_rhct(void)
 {
 	static struct acpi_table_header *rhct;
 	acpi_status status;
@@ -26,7 +26,7 @@ static struct acpi_table_header *acpi_get_rhct(void)
 		}
 	}
 
-	return rhct;
+	return (struct acpi_table_rhct *)rhct;
 }
 
 /*
@@ -48,7 +48,7 @@ int acpi_get_riscv_isa(struct acpi_table_header *table, unsigned int cpu, const
 	BUG_ON(acpi_disabled);
 
 	if (!table) {
-		rhct = (struct acpi_table_rhct *)acpi_get_rhct();
+		rhct = acpi_get_rhct();
 		if (!rhct)
 			return -ENOENT;
 	} else {