extcon: cht-wc: Correct USBID bit field handling

USBID is 2-bit bit field according to specification. Make it clear.

No functional change intended.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>