Optimise GPADC driver:
* for code readability and maintenance by grouping similar cheking
* for performance by grouping several writing to control register
Signed-off-by: Alexandre Bourdiol <alexandre.bourdiol@stericsson.com> Signed-off-by: Lee Jones <lee.jones@linaro.org> Reviewed-by: Philippe LANGLAIS <philippe.langlais@stericsson.com> Acked-by: Samuel Ortiz <sameo@linux.intel.com>