selftests: mlxsw: qos_dscp_bridge: Fix

There are two problems in this test case:

- When indexing in bash associative array, the subscript is interpreted as
  string, not as a variable name to be expanded.

- The keys stored to t0s and t1s are not DSCP values, but priority +
  base (i.e. the logical DSCP value, not the full bitfield value).

In combination these two bugs conspire to make the test just work,
except it doesn't really test anything and always passes.

Fix the above two problems in obvious manner.

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