ixgbe/ixgbevf: Limit maximum jumbo frame size to 9.5K to avoid Tx hangs

This change limits the PF/VF driver to 9.5K max jumbo frame size in order
prevent a possible Tx hang in the adapter when sending frames between
pools.

All of the parts in ixgbe support a maximum frame of 15.5K for standard
traffic, however with SR-IOV or DCB enabled they should be limiting the
MTU size to 9.5K.  Instead of adding extra checks which would have to
change the MTU when we go into or out of these modes it is preferred to
just use a standard 9.5K MTU limit for all modes so that this extra
overhead can be avoided.

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