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

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.

Fixes: 0d15a26b247d ("net: ti: icssg-prueth: Add ICSSG FW Stats")
Signed-off-by: Philippe Schenker <philippe.schenker@impulsing.ch>
Reviewed-by: Simon Horman <horms@kernel.org>
Cc: danishanwar@ti.com
Cc: rogerq@kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260618093037.3448858-1-dev@pschenker.ch
Signed-off-by: Jakub Kicinski <kuba@kernel.org>