nios2: don't leave NULLs in sys_call_table[]

fill the gaps in there with sys_ni_syscall, as everyone does...

Fixes: 82ed08dd ("nios2: Exception handling")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>