net: lan743x: fix potential out-of-bounds write in lan743x_ptp_io_event_clock_get()

net: lan743x: fix potential out-of-bounds write in lan743x_ptp_io_event_clock_get()

Before calling lan743x_ptp_io_event_clock_get(), the 'channel' value
is checked against the maximum value of PCI11X1X_PTP_IO_MAX_CHANNELS(8).
This seems correct and aligns with the PTP interrupt status register
(PTP_INT_STS) specifications.

However, lan743x_ptp_io_event_clock_get() writes to ptp->extts[] with
only LAN743X_PTP_N_EXTTS(4) elements, using channel as an index:

    lan743x_ptp_io_event_clock_get(..., u8 channel,...)
    {
        ...
        /* Update Local timestamp */
        extts = &ptp->extts[channel];
        extts->ts.tv_sec = sec;
        ...
    }

To avoid an out-of-bounds write and utilize all the supported GPIO
inputs, set LAN743X_PTP_N_EXTTS to 8.

Detected using the static analysis tool - Svace.
Fixes: 60942c397af6 ("net: lan743x: Add support for PTP-IO Event Input External Timestamp (extts)")
Signed-off-by: Alexey Kodanev <aleksei.kodanev@bell-sw.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Rengarajan S <rengarajan.s@microchip.com>
Link: https://patch.msgid.link/20250616113743.36284-1-aleksei.kodanev@bell-sw.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>