net/mlx5e: Fix incompatible casting

Device supports setting of a single fec mode at a time, enforce this
by bitmap_weight == 1. Input from fec command is in u32, avoid cast to
unsigned long and use bitmap_from_arr32 to populate bitmap safely.

Fixes: 4bd9d507

 ("net/mlx5e: Enforce setting of a single FEC mode")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>