fsl/fman: remove unused struct member

Remove unused struct member second_largest_buf_size. Also, an out of
bounds access would have occurred in the removed code if there was only
one buffer pool in use.

Signed-off-by: Madalin Bucur <madalin.bucur@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>