mfd: ab8500-debug: Add ADC input channel usb_id in debugfs

Signed-off-by: Yang QU <yang.qu@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Alexandre BOURDIOL <alexandre.bourdiol@stericsson.com>
Reviewed-by: Philippe LANGLAIS <philippe.langlais@stericsson.com>
Reviewed-by: Mattias WALLIN <mattias.wallin@stericsson.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>