extcon: max77693: Remove duplicate code by making function

This patch make max77693-muic_get_cable_type() function to remove
duplicate code because almost internal function need to read
adc/adc1k/adclow/chg_type value of MUIC register. Also, this patch
add description of internal function move field constant of muic device
from extcon-max77693 driver to max77693 header file because of it
is needed for masking some interrupt through platform data.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>