net/mlx5e: Use rx_missed_errors instead of rx_dropped for reporting buffer exhaustion

Previously, the driver incorrectly used rx_dropped to report device
buffer exhaustion.

According to the documentation, rx_dropped should not be used to count
packets dropped due to buffer exhaustion, which is the purpose of
rx_missed_errors.

Use rx_missed_errors as intended for counting packets dropped due to
buffer exhaustion.

Fixes: 269e6b3a ("net/mlx5e: Report additional error statistics in get stats ndo")
Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>