net: lan969x: fix DSM calendar for empty taxis

lan969x_dsm_calendar_calc() computes the DSM calendar schedule of one
taxi. sparx5_config_dsm_calendar() calls it for each taxi, reusing the
same sparx5_calendar_data.

Before this patch, the schedule is only cleared after the n_devs == 0
early return. For a taxi with no active ports, the function writes
SPX5_DSM_CAL_EMPTY only to schedule[0] and leaves the remaining slots
at whatever they held before. For the first taxi that is zero from
kzalloc(), so sparx5_dsm_cal_len() returns a bogus length and
sparx5_dsm_calendar_check() fails, which fails the probe:

  Port 0: distance 20 above limit 0

A later empty taxi inherits the previous taxi's schedule instead, which
passes the check, so only an empty taxi 0 triggers the failure.

Clear the full schedule up front on every path, and on the empty path
place a single idle marker (SPX5_DSM_CAL_MAX_DEVS_PER_TAXI) in slot 0,
so the update step sees a calendar length of one and programs one idle
slot into the hardware, matching what sparx5_dsm_calendar_calc() does.

Reported by a customer whose DT enables none of the ports in a taxi.
Reproduced on lan969x by disabling all taxi 0 ports in the DT.

Fixes: 5d2ba3941016 ("net: lan969x: add function for calculating the DSM calendar")
Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Link: https://patch.msgid.link/20261020-lan969x-dsm-empty-taxi-v1-1-14790f1e7430@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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