selftests: netdevsim: Always initialize 'RET' variable

The variable is used by log_test() to check if the test case completely
successfully or not. In case it is not initialized at the start of a
test case, it is possible for the test case to fail despite not
encountering any errors.

Example:

```
...
TEST: Trap group statistics                                         [ OK ]
TEST: Trap policer                                                  [FAIL]
	Policer drop counter was not incremented
TEST: Trap policer binding                                          [FAIL]
	Policer drop counter was not incremented
```

Failure of trap_policer_test() caused trap_policer_bind_test() to fail
as well.

Fix by adding missing initialization of the variable.

Fixes: 5fbff58e

 ("selftests: netdevsim: Add test cases for devlink-trap policers")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>