hw: timer: ibex_timer: Fixup reading w/o register

hw: timer: ibex_timer: Fixup reading w/o register

This change fixes a bug where a write only register is read.
As per https://docs.opentitan.org/hw/ip/rv_timer/doc/#register-table
the 'INTR_TEST0' register is write only.

Signed-off-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20220110051606.4031241-1-alistair.francis@opensource.wdc.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>