net/mlx5e: Add port FEC get/set functions

Added functions to query and set link FEC policy.
To get/set FEC capabilities in PPLM reg we need to query
current link speed.
'mlx5_get_fec_speed_field' queries current link speed and returns
correct field offset.

FEC Query's return value is divided into 'active FEC policy', which is
the FEC policy used by the link, and 'configured FEC policy', which
is the FEC policy requested by the user.
The two values may differ if:
1) FEC policy was configured to 'auto',
   in which case the active FEC policy would be the default FEC policy
   for current link speed.

2) FEC policy was changed, but no link reset is performed. In which case,
   the active FEC policy would become the configured one after a link
   reset.

FEC set function sets FEC policy for all link speeds and perform link
reset.
1) If current link speed doesn't support requested FEC policy,
   the function fails.
2) If a different link speed doesn't support requested FEC policy,
   FEC capbilities for this speed are turned off and a warning message
   is printed.

Signed-off-by: Shay Agroskin <shayag@mellanox.com>
Reviewed-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>