mfd: pcf50633-adc: Fix potential race in pcf50633_adc_sync_read

Currently it's not guaranteed that request struct is not already freed when
reading from it. Fix this by moving synced request related fields from the
pcf50633_adc_request struct to its own struct and store it on the functions
stack.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>