net/mlx5e: Trim the length of the num_doorbell error

net/mlx5e: Trim the length of the num_doorbell error

When trying to set num_doorbells to a value greater than the max number
of channels, the error message was going over the netlink limit of 80
chars, truncating the most important part of the message, the number of
channels.

Fix that by trimming the length a bit.

Fixes: 11bbcfb7668c ("net/mlx5e: Use the 'num_doorbells' devlink param")
Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/1762681073-1084058-3-git-send-email-tariqt@nvidia.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>