extcon: max8997: Consolidate duplicate code for checking ADC/CHG cable type

This patch make max8997_muic_get_cable_type() function to remove
duplicate code for checking ADC/Charger cable type because almost
internal function need to read adc/chg_type value of MUIC register.

Also, remove *_detach() function, extcon-max8997 driver treat
attach/detach operation of cable in max8997_*_handler() function.
Lastly, this patch move defined constant in header file(include/
linux/mfd/max8997.h, max8997-private.h) because defined constant
is only used in the 'extcon-max8997.c'.

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