net: macb: increase max_mtu for oversized frames

Increase max_mtu from 1500 to 1518 bytes when not configured for jumbo
frames. Use 1536 as a starting point as documented in macb.h for
oversized (big) frames, which is the configuration applied in case
jumbo frames capability is not configured; ref. macb_main.c.

Signed-off-by: Pieter Van Trappen <pieter.van.trappen@cern.ch>
Link: https://patch.msgid.link/20240812090657.583821-1-vtpieter@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>