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: 60942c3 ("net: lan743x: Add support for PTP-IO Event Input External Timestamp (extts)")
Signed-off-by: Alexey Kodanev <[email protected]>
Reviewed-by: Jacob Keller <[email protected]>
Acked-by: Rengarajan S <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

Author: akodanev

drivers/net/ethernet/microchip/lan743x_ptp.h

@@ -18,9 +18,9 @@
  */
 #define LAN743X_PTP_N_EVENT_CHAN	2
 #define LAN743X_PTP_N_PEROUT		LAN743X_PTP_N_EVENT_CHAN
-#define LAN743X_PTP_N_EXTTS		4
-#define LAN743X_PTP_N_PPS		0
 #define PCI11X1X_PTP_IO_MAX_CHANNELS	8
+#define LAN743X_PTP_N_EXTTS		PCI11X1X_PTP_IO_MAX_CHANNELS
+#define LAN743X_PTP_N_PPS		0
 #define PTP_CMD_CTL_TIMEOUT_CNT		50
 
 struct lan743x_adapter;