nios2: dts: Fix tse_mac "max-frame-size" property

The given value of 1518 seems to refer to the layer 2 ethernet frame
size without 802.1Q tag. Actual use of the "max-frame-size" including in
the consumer of the "altr,tse-1.0" compatible is the MTU.

Fixes: 95acd4c7 ("nios2: Device tree support")
Fixes: 61c610ec ("nios2: Add Max10 device tree")
Cc: <stable@vger.kernel.org>
Signed-off-by: Janne Grunau <j@jannau.net>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>