drm/i915/gvt: fix return value in mul_force_wake_write

All mmio handlers should return a negetive value for failure, not 1.
Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>

drivers/gpu/drm/i915/gvt/handlers.c

@@ -220,7 +220,7 @@ static int mul_force_wake_write(struct intel_vgpu *vgpu,
 		default:
 			/*should not hit here*/
 			gvt_err("invalid forcewake offset 0x%x\n", offset);
-			return 1;
+			return -EINVAL;
 		}
 	} else {
 		ack_reg_offset = FORCEWAKE_ACK_HSW_REG;