netdevsim: Ensure policer drop counter always increases

In case the policer drop counter is retrieved when the jiffies value is
a multiple of 64, the counter will not be incremented.

This randomly breaks a selftest [1] the reads the counter twice and
checks that it was incremented:

```
TEST: Trap policer                                                  [FAIL]
	Policer drop counter was not incremented
```

Fix by always incrementing the counter by 1.

[1] tools/testing/selftests/drivers/net/netdevsim/devlink_trap.sh

Fixes: ad188458 ("netdevsim: Add devlink-trap policer support")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>