ixgbevf: Cleanup handling of configuration for jumbo frames

This change moves the code for notifying the PF of the VF maximum packet
size into the vf.c file.  The main motivation behind this is that the vf.c
file is supposed to contain all of the messages used when communicating
with the PF.

In addition it creates a separate function for setting the Rx buffer size
so that we have on centralized area to review what buffer sizes will be
requested by the VF.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>