net: ethernet: cortina: Fix max RX frame define

Enumerator 3 is 1548 bytes according to the datasheet.
Not 1542.

Fixes: 4d5ae32f

 ("net: ethernet: Add a driver for Gemini gigabit ethernet")
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20231109-gemini-largeframe-fix-v4-1-6e611528db08@linaro.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>