tools/hotplug: set mtu from bridge for tap interface

tools/hotplug: set mtu from bridge for tap interface

With changeset 22885 support was added for setting the MTU in the vif-bridge
script for when a vif interface was set to 'online'.  The was not done for the
'add' operation.  The 'add' operation was added to the script for when tap
devices were specified (c/s 21944). With the setting of the MTU for the
'online' case was there a reason for omitting the 'add'?

This patch sets the MTU for both 'online' and 'add' in the vif-bridge script.

Signed-off-by: Charles Arnold <carnold@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>