sfc: ensure type is valid before updating seen_gen

In the case of invalid or corrupted v2 counter update packets,
 efx_tc_rx_version_2() returns EFX_TC_COUNTER_TYPE_MAX.  In this case
 we should not attempt to update generation counts as this will write
 beyond the end of the seen_gen array.

Reported-by: coverity-bot <keescook+coverity-bot@chromium.org>
Addresses-Coverity-ID: 1527356 ("Memory - illegal accesses")
Fixes: 25730d8b ("sfc: add extra RX channel to receive MAE counter updates on ef100")
Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>