extcon: axp288: Add missed error check

It seems from the very beginning the error check has been missed
in axp288_extcon_log_rsi(). Add it here.

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