r8152: fix the warnings and a error from checkpatch.pl

Fix the following warnings and error:
 - WARNING: usb_free_urb(NULL) is safe this check is probably not required
 - WARNING: kfree(NULL) is safe this check is probably not required
 - ERROR: do not use C99 // comments

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>