igb: ensure setting MTU sets also max_frame_size

igb: ensure setting MTU sets also max_frame_size

This is a regression from the out-of-tree Intel IGB driver happened
between 5.3.5.10 and 5.3.5.18.
The condition here should be actually reveresed, but as we always can
be sure to have a MAX/MIN MTU defined we can just remove it,
essentially going back to the previous code state (which also works
with our current 4.15 kernel).

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>