net: dsa: felix: fix incorrect clamp calculation for burst

Currently burst is clamping on rate and not burst, the assignment
of burst from the clamping discards the previous assignment of burst.
This looks like a cut-n-paste error from the previous clamping
calculation on ramp.  Fix this by replacing ramp with burst.

Addresses-Coverity: ("Unused value")
Fixes: 0fbabf87

 ("net: dsa: felix: add support Credit Based Shaper(CBS) for hardware offload")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>