mlxsw: spectrum_pgt: Generalize PGT allocation

PGT blocks are allocated through the function
mlxsw_sp_pgt_mid_alloc_range(). The interface assumes that the caller knows
which piece of PGT exactly they want to get. That was fine while the FID
code was the only client allocating blocks of PGT. However for SW-allocated
LAG table, there will be an additional client: mlxsw_sp_lag_init(). The
interface should therefore be changed to not require particular
coordinates, but to take just the requested size, allocate the block
wherever, and give back the PGT address.

In this patch, change the interface accordingly. Initialize FID family's
pgt_base from the result of the PGT allocation (note that mlxsw makes a
copy of the family structure, so what gets initialized is not actually the
global structure). Drop the now-unnecessary pgt_base initializations and
the corresponding defines.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>