target/riscv: Do not setup pmu timer if OF is disabled

target/riscv: Do not setup pmu timer if OF is disabled

The timer is setup function is invoked in both hpmcounter
write and mcountinhibit write path. If the OF bit set, the
LCOFI interrupt is disabled. There is no benefitting in
setting up the qemu timer until LCOFI is cleared to indicate
that interrupts can be fired again.

Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Message-ID: <20240711-smcntrpmf_v7-v8-12-b7c38ae7b263@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>