hci_usb: remove code obfuscation

_urb_free is an alias for kfree... making code longer & harder to
read. Remove it.

Signed-off-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>