mfd: ab8500-gpadc: Optimise GPADC driver

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>