mlxsw: spectrum_buffers: Split business logic from mlxsw_sp_port_sb_pms_init()

The function is used to set the per-port shared buffer quotas.
Currently, these quotas are only set for front panel ports, but a
subsequent patch will configure these quotas for the CPU port as well.

The configuration required for the CPU port is a bit different than that
of the front panel ports, so split the business logic into a separate
function which will be called with different parameters for the CPU
port.

No functional changes intended.

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