mlxsw: core: Unregister EMAD trap using FORWARD action

The device's manual (PRM - Programmer's Reference Manual) classifies the
trap that is used to deliver EMAD responses as an "event trap". Among
other things, it means that the only actions that can be associated with
the trap are TRAP and FORWARD (NOP).

Currently, during driver de-initialization the driver unregisters the
trap by setting its action to DISCARD, which violates the above
guideline. Future firmware versions will prevent such misuses by
returning an error. This does not prevent the driver from working, but
an error will be printed to the kernel log during module removal /
devlink reload:

mlxsw_spectrum 0000:03:00.0: Reg cmd access status failed (status=7(bad parameter))
mlxsw_spectrum 0000:03:00.0: Reg cmd access failed (reg_id=7003(hpkt),type=write)

Suppress the error message by aligning the driver to the manual and use
a FORWARD (NOP) action when unregistering the trap.

Fixes: 4ec14b76

 ("mlxsw: Add interface to access registers and process events")
Cc: Jiri Pirko <jiri@resnulli.us>
Cc: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/753a89e14008fde08cb4a2c1e5f537b81d8eb2d6.1713446092.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>