mfd: revise locking for pcf50633 ADC

Current implementation is prone to races, this patch attempts to remove all
but one (in pcf50633_adc_sync_read).

The idea is that we need to guard the queue access only on inserting and
removing items. If we insert and there're no more items in the queue it
means that the last irq already happened and we need to trigger ADC
manually. If not, then the next conversion will be triggered by the irq
handler upon completion of the previous.

Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>