extcon: adc-jack: Remove the unneeded num_cables field

This patch removes the 'num_cables' filed from 'struct adc_jack_pdata'
because 'struct extcon_dev' contains the 'max_supported' field which
means the number of supported cable of extcon device.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>