i40e: reinit buffer size each time

When cleaning the ARQ, we must reinitialize the buffer size each time we
go through the loop, because i40e_clean_arq_element returns the message
length in the same field. Without this change, subsequent messages can
be truncated to the length of the previous message.

Change-Id: Ic9c32ff843faf0fc3196d21351a1c3a60c6158eb
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>