Provide an update MTU callback. The function returns -ENOTSUP
if Rx scatter is enabled. Updating the MTU to be greater than
the value configured via the Cisco CIMC/UCSM management interface
is allowed provided it is still less than the maximum egress packet
size allowed by the NIC minus the size of the L2 header.
Signed-off-by: John Daley <johndale@cisco.com>
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