mlxsw: spectrum_buffers: Allocate prs & pms dynamically

Spectrum-2 will be configured with a different set of pools than
Spectrum-1. The size of prs and pms buffers will therefore depend on the
chip type of the device.

Therefore, instead of reserving an array directly in a structure
definition, allocate the buffer in mlxsw_sp_sb_port{,s}_init().

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>