net/mlx5e: Report rx_discards_phy via rx_dropped

We noticed a high number of rx_discards_phy events on certain servers while
running `ethtool -S`. However, this critical counter is not currently
included in the standard /proc/net/dev statistics file, making it difficult
to monitor effectively—especially given the diversity of vendors across a
large fleet of servers.

Let's report it via the standard rx_dropped metric.

Suggested-by: Jakub Kicinski <[e-mail]>
Signed-off-by: Yafang Shao <[e-mail]>
Cc: Saeed Mahameed <[e-mail]>
Cc: Leon Romanovsky <[e-mail]>
Cc: Gal Pressman <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Reviewed-by: Tariq Toukan <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/mellanox/mlx5/core/en_main.c

···
 	}
 
 	stats->rx_missed_errors = priv->stats.qcnt.rx_out_of_buffer;
+	stats->rx_dropped = PPORT_2863_GET(pstats, if_in_discards);
 
 	stats->rx_length_errors =
 		PPORT_802_3_GET(pstats, a_in_range_length_errors) +