ibmveth: Some formatting fixes

IbmVethNumBufferPools -> IBMVETH_NUM_BUFF_POOLS

Also change IBMVETH_MAX_MTU -> IBMVETH_MIN_MTU, it refers to the minimum
size not the maximum.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Santiago Leon <santil@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>