USB: ssb: use devm_kzalloc

Instead of manually handling the frees use devm. There was also a free
missing in the unregister call which is not needed with devm.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>