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>

Makefile

@@ -131,6 +131,7 @@ ${IGBDIR}.prepared: ${IGBSRC}
 	rm -rf ${BUILD_DIR}/${MODULES}/${IGBDIR} $@
 	mkdir -p ${BUILD_DIR}/${MODULES}/${IGBDIR}
 	tar --strip-components=1 -C ${BUILD_DIR}/${MODULES}/${IGBDIR} -xf ${IGBSRC}
+	cd ${BUILD_DIR}/${MODULES}/${IGBDIR}; patch -p1 < ../../../patches/intel/igb/igb_4.15_mtu.patch
 	touch $@
 
 ${IXGBEDIR}.prepared: ${IXGBESRC}