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 <[email protected]>
Reviewed-by: Bin Meng <[email protected]>
Reviewed-by: Alistair Francis <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-id: 20220110051606.4031241[email protected]
Signed-off-by: Alistair Francis <[email protected]>