mlxsw: spectrum_switchdev: Rename MID structure

Currently the structure which represents MDB entry is called
'struct mlxsw_sp_mid'. This name is not accurate as a MID entry stores a
bitmap of ports to which a packet needs to be replicated and a MDB entry
stores the mapping from {MAC, FID} to PGT index (MID).

Rename the structure to 'struct mlxsw_sp_mdb_entry'. The structure
'mlxsw_sp_mid' is defined as part of spectrum.h. The only file which
uses it is spectrum_switchdev.c, so there is no reason to expose it to
other files. Move the definition to spectrum_switchdev.c.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>