The runtime pm resume/suspend will enable/disable pclk (ISI peripheral
clock).
We have to call runtime_pm_get_sync()/runtime_pm_put() when we need to
access ISI registers. In atmel_isi_probe(), remove the isi disable code
as at that moment ISI peripheral clock is not enable yet.
Besides, clock_start()/clock_stop() is used to control the mclk, not
the ISI peripheral clock. So move this to start[stop]_streaming()
function.