Avoid 64 bit divide in cmd_systimer_get().
Avoid losing time when cmn_systimer_counter exceeds PLATFORM_TIMER_SYS_MAX.

Setting cmn_systimer_counter to zero when the systimer exceeded
PLATFORM_TIMER_SYS_MAX would cause time to jump backwards if
cmn_systimer_counter was already greater than PLATFORM_TIMER_SYS_MAX.

So now, instead of zeroing it, we subtract PLATFORM_TIMER_SYS_MAX.