time: fix usec_to_tick()

Commit 8dfafdde88 ("Introduce common timer functions") created a
common definition of usec_to_tick() which had a couple problems:

static unsigned long long usec_to_tick(unsigned long usec)
{
       uint64_t tick = usec * get_tbclk();

That likely overflows.

       usec *= get_tbclk();

That was an attempt to fix it by performing the multiply after the
promotion of usec to 64-bit, but was applied to the wrong variable,
which was never used.

This patch fixes these issues. A user-visible symptom of the problem was
the e.g. "dhcp zImage" using an ASIX USB Ethernet dongle would print:

Waiting for Ethernet connection... unable to connect.

... with no delay before "unable to connect". There are likely other
symptoms.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>

lib/time.c

@@ -71,8 +71,8 @@ unsigned long __weak notrace timer_get_us(void)
 }
 static unsigned long long usec_to_tick(unsigned long usec)
 {
-	uint64_t tick = usec * get_tbclk();
-	usec *= get_tbclk();
+	uint64_t tick = usec;
+	tick *= get_tbclk();
 	do_div(tick, 1000000);
 	return tick;
 }