net: ethernet: ti: icssg: guard PA stat lookups

In the Linux kernel, the following vulnerability has been resolved: net: ethernet: ti: icssg: guard PA stat lookups icssg_ndo_get_stats64() unconditionally calls emac_get_stat_by_name() with FW PA stat names regardless of whether the PA stats block is present on the hardware. emac_get_stat_by_name() already guards the PA stats lookup with `if (emac->prueth->pa_stats)`; when that pointer is NULL the lookup falls through to netdev_err() and returns -EINVAL. Because ndo_get_stats64 is polled regularly by the networking stack this produces thousands of log entries of the form: icssg-prueth icssg1-eth end0: Invalid stats FW_RX_ERROR A secondary consequence is that the int(-EINVAL) return value is implicitly widened to a near-ULLONG_MAX unsigned value when accumulated into the __u64 fields of rtnl_link_stats64, silently corrupting the rx_errors, rx_dropped and tx_dropped counters reported by `ip -s link`. Every other PA-aware code path in the driver is already guarded with the same `if (emac->prueth->pa_stats)` check. Apply the same guard here.