ibmvnic: Fix initial MTU settings

In the current driver, the MTU is set to the maximum value
capable for the backing device. This decision turned out to
be a mistake as it led to confusion among users. The expected
initial MTU value used for other IBM vNIC capable operating
systems is 1500, with the maximum value (9000) reserved for
when Jumbo frames are enabled. This patch sets the MTU to
the default value for a net device.

It also corrects a discrepancy between MTU values received from
firmware, which includes the ethernet header length, and net
device MTU values.

Finally, it removes redundant min/max MTU assignments after device
initialization.

Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>