staging: android: logger: clarify non-update of w_off in do_write_log_from_user

Add comment to explain when w_off is not updated in case of failed second
fragment copy to buffer.

Signed-off-by: Tim Bird <tim.bird@am.sony.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Change-Id: I12bf0447dc87d4628403496e47abe27c4a8b6c20