ACPICA: exserial: don't forget to handle FFixedHW opregions for reading

The initial commit that introduced support for FFixedHW operation
regions did add a special case in the AcpiExReadSerialBus If, but
forgot to actually handle it inside the switch, so add the missing case
to prevent reads from failing with AE_AML_INVALID_SPACE_ID.

Link: acpica/acpica#998
Fixes: ee64b82 ("ACPICA: Add support for FFH Opregion special context data")
Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
Link: https://patch.msgid.link/20250401184312.599962-1-d-tatianin@yandex-team.ru
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: d-tatianin

drivers/acpi/acpica/exserial.c

@@ -201,6 +201,12 @@ acpi_ex_read_serial_bus(union acpi_operand_object *obj_desc,
 		function = ACPI_READ;
 		break;
 
+	case ACPI_ADR_SPACE_FIXED_HARDWARE:
+
+		buffer_length = ACPI_FFH_INPUT_BUFFER_SIZE;
+		function = ACPI_READ;
+		break;
+
 	default:
 		return_ACPI_STATUS(AE_AML_INVALID_SPACE_ID);
 	}