target/openrisc: Remove dead code attempting to check "is timer disabled"

target/openrisc: Remove dead code attempting to check "is timer disabled"

In the mtspr helper we attempt to check for "is the timer disabled"
with "if (env->ttmr & TIMER_NONE)".  This is wrong because TIMER_NONE
is zero and the condition is always false (Coverity complains about
the dead code.)

The correct check would be to test whether the TTMR_M field in the
register is equal to TIMER_NONE instead.  However, the
cpu_openrisc_timer_update() function checks whether the timer is
enabled (it looks at cpu->env.is_counting, which is set to 0 via
cpu_openrisc_count_stop() when the TTMR_M field is set to
TIMER_NONE), so there's no need to check for "timer disabled" in the
target/openrisc code.  Instead, simply remove the dead code.

Fixes: Coverity CID 1005812
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Stafford Horne <shorne@gmail.com>
Message-id: 20201103114654.18540-1-peter.maydell@linaro.org