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

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

Signed-off-by: Bo Liu <liubo03@inspur.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230206081641.3193-1-liubo03@inspur.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>