extcon: Use sysfs_emit() to instead of sprintf()

Follow the advice of the Documentation/filesystems/sysfs.rst that
show() should only use sysfs_emit() or sysfs_emit_at() when formatting
the value to be returned to user space.

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