net: mvneta: fix error message when MTU too large for XDP

The error message says that "Jumbo frames are not supported on XDP", but
the code checks for mtu > MVNETA_MAX_RX_BUF_SIZE, not mtu > 1500.

Fix this error message.

Signed-off-by: Marek Behún <kabel@kernel.org>
Fixes: 0db51da7

 ("net: mvneta: add basic XDP support")
Cc: Lorenzo Bianconi <lorenzo@kernel.org>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Link: https://lore.kernel.org/r/20210105172333.21613-1-kabel@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>