net: stmmac: selftests: Always use max DMA size in Jumbo Test

Although some XGMAC setups support frames larger than DMA size, some of
them may not. As we can't know before-hand which ones support let's use
the maximum DMA buffer size in the Jumbo Tests.

User can always reconfigure the MTU to achieve larger frames.

Fixes: 427849e8

 ("net: stmmac: selftests: Add Jumbo Frame tests")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>