drivers: net: netdevsim: fix devlink_trap selftests failing

devlink_trap tests for the netdevsim fail due to misspelled
debugfs file name. Change this name, as well as name of callback
function, to match the naming as in the devlink itself - 'trap_drop_counter'.

Test-results:
selftests: drivers/net/netdevsim: devlink_trap.sh
TEST: Initialization                                                [ OK ]
TEST: Trap action                                                   [ OK ]
TEST: Trap metadata                                                 [ OK ]
TEST: Non-existing trap                                             [ OK ]
TEST: Non-existing trap action                                      [ OK ]
TEST: Trap statistics                                               [ OK ]
TEST: Trap group action                                             [ OK ]
TEST: Non-existing trap group                                       [ OK ]
TEST: Trap group statistics                                         [ OK ]
TEST: Trap policer                                                  [ OK ]
TEST: Trap policer binding                                          [ OK ]
TEST: Port delete                                                   [ OK ]
TEST: Device delete                                                 [ OK ]

Fixes: a7b3527a

 ("drivers: net: netdevsim: add devlink trap_drop_counter_get implementation")
Signed-off-by: Oleksandr Mazur <oleksandr.mazur@plvision.eu>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>