ixgbe: increase DCB BW calculation for MTU

Change max credit and credit refill to a maximum possible value, 9128.
Too small values cause the incorrect calculation of the bandwidth limits
to each traffic class for frames larger than 4088 bytes.

Signed-off-by: Radoslaw Tyl <radoslawx.tyl@intel.com>
Reviewed-by: Piotr Skajewski <piotrx.skajewski@intel.com>
Reviewed-by: Slawomir Mrozowicz <slawomirx.mrozowicz@intel.com>
Reviewed-by: Alice Michael <alice.michael@intel.com>
Tested-by: Piotr Skajewski <piotrx.skajewski@intel.com>

Obtained from: DPDK (440823f)
MFC after: 1 week