ixgbe: Fill at least min credits to a TC credit refills

Currently credit_refill and credit_max could be zero for a TC and that
is causing Tx hang for CEE mode configuration, so to fix that have at
min credit assigned to a TC and that is as what IEEE mode already does.

Change-ID: If652c133093a21e530f4e9eab09097976f57fb12
Signed-off-by: Vasu Dev <vasu.dev@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>