clocksource/drivers/pistachio: Fix wrong calculated clocksource read value

clocksource/drivers/pistachio: Fix wrong calculated clocksource read value

Let's assume the counter value is 0xf0000000, the pistachio clocksource
read cycles function should return ~0x0fffffff but actually it returns
0xffffffff0fffffff.

That occurs because:

	~(cycle_t)value is different from (cycle_t)~value.

unsigned long val = ~(unsigned long)0xf0000000;
40049a:       48 b8 ff ff ff 0f ff    movabs $0xffffffff0fffffff,%rax

unsigned long val = (unsigned long)~0xf0000000;
40049a:       48 c7 45 f8 ff ff ff    movq   $0xfffffff,-0x8(%rbp)

We fix this issue by calculating bitwise-not counter, then cast to
cycle_t.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>