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>

tools/testing/selftests/drivers/net/netdevsim/devlink_trap.sh

@@ -264,6 +264,8 @@ trap_policer_test()
 	local packets_t0
 	local packets_t1
 
+	RET=0
+
 	if [ $(devlink_trap_policers_num_get) -eq 0 ]; then
 		check_err 1 "Failed to dump policers"
 	fi
@@ -328,6 +330,8 @@ trap_group_check_policer()
 
 trap_policer_bind_test()
 {
+	RET=0
+
 	devlink trap group set $DEVLINK_DEV group l2_drops policer 1
 	check_err $? "Failed to bind a valid policer"
 	if [ $(devlink_trap_group_policer_get "l2_drops") -ne 1 ]; then