openrisc: Fix up audit_syscall_[entry|exit]() usage

Commits d7e7528b and
b05d8447 simplified the usage of the
audit_syscall_[entry|exit] functions.  Unfortunately, the OpenRISC
architecture didn't get fixed up along with the other architectures when
those patches were pushed.  This makes the relevant changes to this
architecture.

Signed-off-by: Jonas Bonn <jonas@southpole.se>