clocksource: sirf: Disable counter before re-setting it

According to HW spec, we have to disable the counter before setting
it, if we don't this, in pressure test, sometimes the timer might
not generate interrupt any more.

And this patch also fixes a typo for register set by changing 0x7
to 0x3. 0x7 is loop mode in HW, but here we are using oneshot 0x3.

Signed-off-by: Hao Liu <Hao.Liu@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>