mlxsw: spectrum_buffers: Keep mlxsw_sp_sb_prs in mlxsw_sp_sb_vals

The SBPR register configures shared buffer pools. The default
configuration depends on the chip type. Therefore keep it in struct
mlxsw_sp_sb_vals. Redirect the one reference from the global array to
the field.

Because the pool descriptor ID is implicit in the ordering of array
members, both this array and the pool descriptor array have the same
length. Therefore reuse mlxsw_sp_sb.pool_dess_len for the purpose of
determining the length of SBPR array.

Drop the now useless MLXSW_SP_SB_PRS_LEN.

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