net/mlx5e: Extract checking of FEC support for a link mode

The check of whether a given FEC mode is supported in a given link mode
is about to get more complicated, so extract it in a separate function
to avoid code duplication.

Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://lore.kernel.org/r/20240404173357.123307-2-tariqt@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>