nios2: traced syscall does need to check the syscall number

all checks done before letting the tracer modify the register
state are worthless...

Fixes: 82ed08dd

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