extcon: cht-wc: Return from default case to avoid warnings

When we have first case to fall through it's not enough to put
single comment there to satisfy compiler. Instead of doing that,
return fall back value directly from default case.

This to avoid following warnings:

drivers/extcon/extcon-intel-cht-wc.c: In function ‘cht_wc_extcon_get_charger’:
include/linux/device.h:1420:2: warning: this statement may fall through [-Wimplicit-fallthrough=]
  _dev_warn(dev, dev_fmt(fmt), ##__VA_ARGS__)
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/extcon/extcon-intel-cht-wc.c:148:3: note: in expansion of macro ‘dev_warn’
   dev_warn(ext->dev,
   ^~~~~~~~
drivers/extcon/extcon-intel-cht-wc.c:152:2: note: here
  case CHT_WC_USBSRC_TYPE_SDP:
  ^~~~

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